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tuned to add so important a discovery to our knowledge of
the laws of light was compelled to spend his boyhood in
the drudgery of a manufactory, and in a capacity which had nothing congenial to his tastes. The little leisure however which The had was spent in the acquisition of a varied circle of knowledge. Besides the study of mathematics and physics, to the latter of which his reputation is chiefly due, he studied anatomy and natural history in general, on one hand, and theology and ecclesiastical history on the other. In furtherance of this diversified class of subjects, which, considering the toil to which the day was devoted, was sufficiently extensive, he undertook the Greek and Roman classics; he was partially acquainted with several of the modern languages, but with French, German, and Italian, he was intimately conversant. It is very rare to see the o union of great powers of reasoning, of memory, and of observation, that was displayed by this eminent Inan. Notwithstanding the cares of a family and the duties which it imposed upon him, Dollond still found means to cultivate the sciences; and having o his eldest son, Peter, to an optical instrument maker, he was in due time able to establish him in business in Vine Court, Spitalfields. In this business he finally joined his son, for the especial purpose, it would seem, of being able to unite his tastes with his business more perfectly than silk weaving enabled him to do. Immediately on this arrangement being completed, Dollond commenced a series of experiments on the dispersion of light, and other subjects connected with the improvement of optical instruments, and especially of telescopes and microscopes, the results of which were communicated to the Royal Society in a series of papers. Three of them were printed in the Philosophical Transactions for 1753, one in 1754, and the last in 1758, the titles of which are given below. It was about 1755 that he entered upon a systematic course of experiments on dispersion, and after, to use his own words, “a resolute perseverance' for more than a year and a half, he made the decisive experiment which showed the error of Newton's conclusions on this subject. [LIGHT.] The memoir in which the series of investigations was detailed appeared in the Philosophical Transactions, and was the last which he gave to the world. It was rewarded by the council of the Royal Society with the Copley medal It was the lot of Dollond to undergo considerable annoy. ance on account of the claims set up for this discovery in favour of others, especially of Euler; but there is not a shadow of a doubt of Dollond's priority as well as originality, in this very important discovery, left on the minds of the scientific world. The discrepancies which followed the application of Newton’s doctrine to the varied cases that presented themselves in the course of different experiments might, in speculative minds, have created a suspicion of the accuracy of that doctrine; yet there does not appear to have been the least hesitation among scientific men in attributing these discrepancies to errors of observation erclusively, and o not the least ground for honestly attempting to deprive Dollond of the honour of the discovery. In the beginning of the year 1761 Dollond was elected a Fellow of the Royal Society, and appointed optician to the king. He did not long survive to enjoy the honour or advantages of his discoveries; as, on the 30th of September of that year, he was attacked by a fit of apoplexy, brought on by a too close and long continued application to a paper which he was studying. This attack immediately deprived him of speech, and in a few hours of life itself. Besides his eldest son Peter, already mentioned, he left another son and three daughters. The two sons carried on the business jointly with great reputation and success; and upon the death of the younger, it went into the hands of a nephew, who took the family name, and who still carries it on without diminution of the high character attached to the name of Dollond. Mr. Dollond's appearance was somewhat stern, and his address and language impressive; but his manners were cheerful, kind, and affable. He adhered to the religious doctrines of his father, and regularly attended the French Protestant Church, of which his life and conversation rendered him a bright ornament. The following is the list of Dollond's published papers: —
1 A better to M. James Short, F.R.S., concerning an
Improvement in Reflecting Telescopes; Phio. Trans., 1753, p. 103 2. Letter to James Short, A.M., F.R.S., concerning a mistake in Mr. Euler's Theorem for correcting the Aberration in the Object Glasses of RefractingTelescopes; Phil. Trans., 1753, p. 287. 3. A description of a Contrivance for measuring Small Angles; Phil. Trans., 1753, p. 178. 4. An Explanation of an Instrument for measuring Small Angles; Phil. Trans., 1754, p. 551. 5. An account of some experiments concerning the dif. ferent Refrangibility of Light; Phil. Trans, 1758, p. 733. DOLOMIEU, DEODAT-GUY-SILVAIN TANCRE. DE DE, was born at Grenoble on the 24th of June, 1750. In early youth he was admitted a member of the religious order of Malta, but in consequence of a quarrel with one of his companions, which ended in a duel fatal to his adversary, he received sentence of death, but, after imprisonment, he was pardoned, and went to France. After some hesitation whether he should devote himself to classical literature or to natural history, he decided in favour of the latter. . While at Metz with the regiment of carbineers, in which he had obtained a commission, he formed an acquaintance with the celebrated and unfortunate La Rochefoucault, which ceased but with his existence; and the attachment for science, by which this nobleman was distinguished probably contributed to confirm Dolomieu in the choice of the pursuit which he had previously made. He was soon afterwards elected a corresponding member of the Academy of Sciences, and quitted the military profession. At the age of twenty-six he went to Sicily, and his first labour was an examination of the environs and strata of AEtna. He next visited Vesuvius, the Appenines, and the Alps, and in 1783 published an account of his visit to the Lipari islands. He returned to France at the commencement of the Revolution, and early ranged himself on the side of liberty. He had however no public employment until the third year of the republic, when he was included in the Ecole de Mines, then established; and he was one of the original members of the National Institute, founded about the same time. He was indefatigable in the pursuit of geological and mine ralogical science, and in less than three years he published twenty-seven original memoirs; among which were those on the nature of leucite, peridot, anthracite, pyroarene, &c When Bonaparte undertook the conquest of Egypt, Dolo mieu accompanied the expedition; on the arrival of which he visited Alexandria, the Delta, Cairo, the Pyramids, and a part of the mountains which bound the valley of the Nile He proposed also to explore the more interesting parts of the country; but before . could carry his plan into execution his health became so deranged that he was compelled to return to Europe. On his passage home he was, with his friend Cordier, the mineralogist, and many others of his countrymen, made prisoner after being driven into the Gulf of Tarentum, and confined in a miserable dungeon. His companions were soon set at liberty, but the remembrance of the disputes which had existed between him and the members of the Order of Malta led to his removal and subsequent imprisonment at Messina, where he was con fined in a dungeon lighted only by one small opening, which, with barbarous precaution, was closely shut every night. The heat, and the small quantity of fresh air admitted by the window of his prison, compelled him to spend nearly the whole of his time in fanning himself with the few tattered remnants of his clothes, in order to increase the circulation of the air. Great exertion and urgent demands were made by the scientific men of various countries to obtain his enlargement; and when, after the battle of Marengo, peace was made with Naples, the first article of the treaty was a stipulation for the immediate release of Dolomieu. On the death of Daubenton he was appointed professor of mineralogy, and soon after his return to France he delivered a course of lectures on the philosophy of mineralogy at the Museum of Natural History. In a short time he again quitted Paris, visited to Alps, and returned to Lyon by Lucerne, the gloo. f lindelwald and Geneva, and from thence to C. here' !. visit his sister and his brother-in-law De P. proved rati was unfortunately attacked by a disorder whic in the 53rd year of his age.
his vast st He had projected two journeys - Oro
of go. knowledge, the first through Germany, and the second through Norway, Denmark, and Sweden. He also proposed to publish a work which he had planned in his prison at Messina; of this he printed a fragment on Mineral Species, which is a monument at once of his misfortunes and his genius, being written in his dungeon in Sicily, on the margin of a few books with a bone sharpened against his prison walls for a pen, and the black of his lampsmoke mixed with water for ink. In this work the author proposes that the integral molecule shall be regarded as the principle by which #. is to be determined, and that no other specific characters should be admitted than those which result from the composition or form of the integral molecule. It must however be admitted as an objection to this proposal that the integral molecule is not always easily ascertained or characterized. “From a careful perusal of the works of Dolomieu, observes Dr. Thomson, Annals of Philosophy, vol. xii., p. 166, ‘especially his later ones, the following appear to §: the results of his observations and the bases of his geological system : “It appears highly probable, from geometrical considerations and from the theory of central forces, that the earth at the time when it received its spheroidal shape was in a state of fluidity. This fluidity was probably neither the result of igneous fusion nor of aqueous solution, but of the intermixture of a substance or substances with the earthy particles fusible, like sulphur, at a moderate heat, capable of entering into more rapid combustion when exposed to the air, decomposing water, and involving the gas thus produced so as to enter into strong effervescence when the superincumbent pressure does not exceed a given quantity. “The surface of this fluid, by the action of the air on the combustible ingredient which occasioned its fluidity, would at length become consolidated, and would envelop the whole spheroid with a shell of less specific gravity than the fluid part, and therefore floating securely on its surface; this latter essential condition being rendered extremely probable from the well-known fact, that the mean specific gravity of the globe is considerably greater than that of any natural rock hitherto known. “The interposition of this solid shell of stony matter, a bad conductor of heat, between the liquid and gaseous portions of the globe, would enable the aqueous and other easily-condensible vapours to separate themselves from the permanently-elastic gases, and thus the matter of the globe would be arranged in four concentric spheroids according to their respective gravities: namely, the liquid central portion, the solid stony, the liquid *. and the permanently elastic. As the water penetrated through the stony portion to the nearest fluid part, it would be gradually decomposed, the consolidation would proceed downwards, the newly consolidated part would enlarge in bulk, and thus, aided by the elastic expansion of the hydrogenous base of the decomposed water, would occasion rifts of greater or less magnitude in the superincumbent mass. Some of the larger of these rifts would open a free communication between the ocean and the fluid central mass, a torrent of water would rush down, and the effervescence occasioned by its decomosition would produce the first submarine volcanos. The ava thus ejected would in time raise the mouth of the volcano above the surface of the water, when it would either become quiescent, or, if supplied laterally with a sufficient quantity of water, would assume the character of a proper volcano, or burning mountain. The secondary rocks, i.e. all those which either themselves contain organic remains or are associated with those which do, were deposited from solution or suspension in water. By the deposition of these, and the increase by consolidation of the primitive rocks, the thickness of the mass incumbent above the central fluid is continually increasing; and those causes which antiently broke through the solid crust of the globe are now rarely able to produce the same effect; hence the greater magnitude and frequency of volcanic eruptions in the earliest ages of the earth ; for the same reason the elevation of large mountainous or continental tracts above the general level no longer takes place; and thus the surface of the globe has become a safe and proper habitation for man and other animals. If the land animals were created as early as possible, that is, while the great changes of the earth's surface abovementioned were still in process, many of the most antient tra;" of deluges and other catastrophes may be founded on t.
“The fluidity of the central part of the globe, and its eonnectiou with the active volcanos, affords a plausible theory of earthquakes, and particularly accounts for the propagation of §. shock, with diminishing intensity, to great distances. “The crystals of hornblende, of felspar, &c., which occur so abundantly in most lavas, are, according to this theory, net those component ingredients of rocks which have re sisted the heat while the other substances associated with them have been melted; nor are they the result of the slow cooling of a vitreous mass, but are produced by crystallization in the central fluid, and are accumulated, on account of their inferior specific gravity, about its surface, together with the peculiar inflammable matter in which they float, whence they are disengaged during volcanic eruptions.” DOLOMITE, a variety of magnesian limestone first noticed by Dolomieu. It occurs mostly massive, and in mountain masses; it is usually white, sometimes greyish or yellowish; its structure is sometimes slaty; it is frequently translucent on the edges. It is softer than common limeStone. The Apennines are partly composed of dolomite, and it occurs at Iona. Sometimes it is met with in veins accompanied by quartz, carbonate of lime, &c. The dolomite of the Apennines consists of 59 carbonate of lime and 40 carbonate of magnesia: it contains a variable quantity of oxide of iron. Compact Dolomite or Gurhoffan is snow white, and very compact. The surface, when newly broken, is scarcely shining, and the fragments, which are sharp, are translucent on the edges; the fracture is flat conchoidal, and its hardness is considerable. It occurs in veins traversing serpentine between Gurhoff (whence its name) and Aggsbach, in Lower Austria. According to Klaproth, it consists of carbonate of lime 70.50, and carbonate of magnesia 29.50. DOLPHIN. [Whales.] DOMBES, a principality in France, to the east of the river Saône; one of the divisions existing before the Revolution. It consisted of two portions separated from each other by an intervening part of the district of Bresse by which the eastern portion was entirely surrounded. The western portion was bounded on the west by Lyonnois, Beaujolais, and Maconnois, from which it was separated by the river Saône; on the south, by the districts of Franc-Lyonnois and Bresse; and on the north and east by Bresse. It is now comprehended in the department of the Ain. It contained seven towns, among which were Trévoux, the capital, and Thoissey. Dombes was governed by sovereign princes of its own, who derived a considerable revenue from it, until the year 1762, when the reigning prince exchanged his o for the duchy of Gisors in Normandy, and other lands. Dombes was united to the crown; but retained its ‘parlement,” or local civil court. DO'MBEYA, a name given by botanists to a Sterculiaceous genus of shrubs or trees inhabiting the East Indies and the Isles of France, Bourbon, and Madagascar. They have a five-parted persistent calyx, surrounded by a threeleaved unilateral involucel. The petals are five. The stamens are from fifteen to twenty, scarcely monadelphous, five of them being sterile, with from two to three fertile ones between each sterile stamen. The name Dombeya was also applied to the plant now called Araucaria earcelsa. The mathematical theory of a dome, so far as considerations requisite for security are concerned, is more simple than that of an arch. Imagine two vertical planes passing through the axis of a dome, and making a small angle with each other. These planes intercept (as in the cut) two symmetrically opposite slices of the dome, which tend to support each other at the crown. This support might be made complete and effectual upon principles explained in the article ARCH; so that in fact each small slice of the dome, with its opposite, might compose a balanced arch. Any slice of such a dome is supported by the opposite one only, so that all the rest might be taken away. Now suppose such a dome to be constructed upon an interior centering, of which however the arches are not separately balanced, in consequence of the weight of A P K being so great that the resultant of this weight and the horizontal thrust at A falls obliquely, not being, as in a balanced arch, perpendicular to PK, but cutting the line KP produced towards the axis. Still this dome cannot fall: for since every part of the horizontal course of stones has the same tendency to fall inwards, these pressures inwards cannot produce any effect, except a lateral pressure of each slice upon the two which are vertically contiguous. Hence the condition of equilibrium of a dome is simply this, that the weight of any portion A M PK must be too great for a balanced arch. Upon this same principle a dome may even be constructed with a concave exterior: and in a dome of convex exterior a portion of the crown may be removed, as is the case when the building is surmounted by a lantern. The tendency of the upper part to fall inwards being equal all round, each stone is supported by those adjacent. From the preceding it appears that it would be (in comparison with an arch) easy to construct a dome with perfectly polished stones, and without cement. The friction of the stones and the tenacity of the cements are of course additional securities. The part in which the construction is weakest will be near the base, more particularly if the joints become nearly horizontal at the base, or if the circumference at the base be very considerable. This weak point is generally secured in practice by bringing strong chains or hoops round the horizontal courses at the interior of the base. Dr. Robison says “The immense addition of strength which may be derived from hooping largely compensates for all defects; and there are hardly any bounds to the extent to which a very thin dome vaulting may be carried when it is hooped or framed in the direction of the horizontal courses.’ This system of internal hooping is every way preferable to reliance upon cements, and may, without interference with the ornamental part of the design, be carried to any length. Among other advantages, a dome may be made by means of it to rise vertically from the base, which cannot be the case in an arch. The thickness of a dome should increase towards the base. A perfectly spherical dome, that is, a segment of a hollow shell cut off by a plane, and therefore of uniform thickness, will stand securely if the arch of the generating circle subtend at the centre less than 51° 49'. The law of the thickness necessary to seeure equilibrium is as follows: A
Let the dome be formed by the revolution of AV and B W, and let PK, the joint of one of the stones, be always perpendicular to the interior curve; which is usually the case in practice. Let A M = a, M P = y, P K = 2, arc B P = s : and let p be any constant is.” than unity, and A any oomstant whatever. Then there will be equilibrium, the equation of B P W being given, if Ap/dr Y" d dr y \dy) ds dy or pbeing the angle KGB, and p the radius of curvature at
z - o py cost e
For the demonstration of this formula, see Venturoli's Me chanics (Creswell's translation), or Robison's Mechanical Philosophy. It is not necessary that p should be a constant: a reference to the work first cited will show how to proceed on the supposition that it is a function of a greater than unity. DOME, a term applied to a covering of the whole or part of a building. The Germans call it Dom, and the Italians Duomo, and apply the term to the principal church of a city, although the building may not have any spherical or polygonal dome. From this and other circumstances we may infer the term to be derived from the Latin Domus, house. The remains of antient domes are generally spherical in their form, and built of stone or tufo. The word dome is applied to the external part of the spherical or polygonal roof, and cupola to the internal part. Cupola is derived from the Italian cupo, deep, whence also our word cup. But cupola and o are often used synonymously, although perhaps incorrectly. Ruins of numerous domes still exist in the neighbourhood of Rome and Naples. The principal in and near Rome are the Pantheon and the temples of Bacchus, Vesta, Romulus, Hercules, Cybele, Neptune, and Venus, and also some of the Chambers of the Thermae. The most magnificent dome of antiquity is that of the Pantheon, supposed to be a chamber of the great baths of Agrippa. The diameter of the dome internally is 142 ft. 8% in., with a circular opening at the top in the centre 28 ft. 6 in. in diameter. The height of the dome from the top of the attic is 70 ft. 8 in. Internally it is decorated with five rows of square compartments. Each row is considerably larger than that immediately above it, as they converge towards the top. The large squares, all of which are rather more than 12 feet each way, contain four smaller squares sunk one within the other. It is supposed that these squares were decorated with plates of silver, from some fragments of that metal having been found on them. The opening at the top of the dome was decorated with an ornamented bronze moulding, gilt. The external part of the dome appears also to have been decorated with bands of bronze. Constantius II. removed the silver and bronze with which the building was decorated. The base of the dome externally consists of a large plinth with six smaller plinths or steps above it; and in the curve of the dome a flight of steps is formed which leads to the opening at the top of the dome. From the drawings of the architect Serlio it appears that flights of steps were formed at intervals all round the dome, which are now covered with the lead placed there by order of Urban VIII. The dome is constructed of bricks and rubble. Sunk bands round the hollow squares or caissons appear to be formed in brick, and the other parts in tufo and pumice stone. The thickness of the dome of the Pantheon is about 17 ft. at the base, 5 ft. 14 in. at the top of the highest step, and 4 ft. 7 in. at the top of the dome. The circular wall which supports the dome is 20 ft. thick. This wall is however divided by several large openings, and is furnished with dis charging arches of brick. It is most probable that the dome of the Pantheon was executed by means of a centering of wood with the hollow squares formed in relief upon it, as was afterwards done in constructing the great vaulting of St. Peter's. The dome of one of the chambers of the Thermae of Caracalla was 111 feet in diameter. In the Thermae of Titus there are two domes each 84 feet in diameter, and in the baths of Constantine there was one of 76 feet. There were three domes in the baths of Diocletian, of which two still remain; one is 73 feet 6 inches in diameter, and the other 62 feet 3 inches. Judging from those that remain, there is every reason to believe that in the Thermae they were all lighted from above, like the dome of the Pantheon. Near Pozzuoli there is a very perfect circular building, with a dome 96 feet in diameter, built of volcanic tufo and pumice stone. The temple of Minerva Medica, without the walls of Rome, was on the plan a polygonal dome of ten sides built of brick and pumice stone. This building does not appear to have had any opening at the top. The antients appear to have constructed domes on corbels. At Catania there is a spherical dome which covers a square vestibule; and in one of the octagonal rooms of the enclosure surrounding the baths of C*** la the corbels still remain which most probably supported the dome of the chamber.
The dome of Santa Sophia, at Constantinople, built in the
Parallel section of the four principal Domes of Europe, to the same scale; by Joseph Gwilt; published by Priestley and Weale, High Street, Bloomsbury. (With the permission of the Publishers.)
reign of Justinian, is the most remarkable and the earliest constructed after those of the Romans. Anthemius of Tralles and Isidorus of Miletus were the architects. The present dome, however, was reconstructed by the nephew of Isidorus. It rests on the square formed at the intersection of the arms of the Greek cross: the diameter is about 111 feet, and the dome 40 feet high. The dome is supported by four corbellings placed in the angles of the square. The corbels are surmounted by a kind of cornice which supports a circular gallery. The lower part of the dome is pierced with a row of small windows adorned with columns on the exterior. Externally the dome is divided by projecting ribs, rounded and covered with lead. The top is surmounted by a lantern or finishing like a baluster, on which is a cross. The dome of Anthemius and Isidorus was not so high, and was partly destroyed twenty-one years after its construction by an earthquake during the lifetime of Justinian. In the reconstruction the nephew of Anthemius used very light white bricks, only one fifth the weight of common bricks, which are said to have been made in Rhodes. It appears from the history and description of the building of Santa Sophia, by Procopius, that the architects encountered many difficulties, which arose probably from not being thoroughly acquainted with the principles on which domes should be constructed. (Procopius, tropi wrouárov, lib. i. cap. 1.) The dome of San. Vitale, at Ravenna, which is considered to be more antient than that of Santa Sophia, is curiously constructed. The hower part of the plan of the dome is a regular octagon, which is supported }. eight piers placed at the angles of the dome. Between these angles are seven tall niches divided into two stories. The lower part of these niches is open, and ornamented with columns, like Santa Sophia. The eighth side of the dome is pierced with a great arch forming an entrance. This arch is of the same diameter and the same elevation as the niches. The
* > / ... . . . . . --
wall above the niches and arch, which is without openings, sustains a hemispherical dome, the plan being a circle described within a regular octagon. Corbels are not employed as at Santa Sophia, but the arches support the gathering over, or corbelling, which forms the circular base of the dome. The base of the dome is pierced with eight windows, each divided in the middle by a column which supports two small arches. The dome itself is built with a double row of pipes, hollow at one end and pointed at the other, the point of one being placed in the hollow of the preceding. They are thus continued in a gentle spiral line until they finish at the top. Between the top of the small arched windows and the pipes there is a construction formed with vases, not unlike the system adopted in the circus of Caracalla. [CIRCUs, vol. vii., p. 197.] The dome itself is covered with mortar both within and without. The church of San Marco at Venice, built in the tenth century, by order of Pietro Orseolo, the then doge, is decorated with five domes. One of these, placed in the centre of the church, is much larger than the others. Each dome is enclosed within four pieces of semi-cylindrical vaulting, together forming a square, in the angles of which are four corbels, which gather in the circular base of each dome. The lower part of the dome is pierced with small windows. The interior is covered with mosaic, and the top of the dome is terminated with a finishing on which is a cross. In 1523 the doge, Andrea Gritti, caused the domes to be repaired, and Sansovinus, the architect, restored in a great measure the supports, and placed (at about one third of its height) a great circle of iron round the large dome to prevent its falling; a precaution which has been completely successful. The other domes are not so well preserved. In 1729 one of the smaller domes was in danger of falling, from the decay which had taken place in a circular bond placed at the base of the dome. Stone was however substituted for
the wooden bond, and a circle of iron placed without the dome near its base. In 1735 Andrew Tirali, the architect to the church, placed an iron circle round the dome which is near the great gate, on account of some small fractures which were then perceived. If, however, the other domes are constructed with a wooden bond, it is very probable that they will eventually fall unless steps be taken in time to remove the timber. By the use however of corrosive sublimate, now used in Kyan's patent for preserving wood from the dry rot, wood may be used in the construction of domes with much more security as regards durabilitv. #. celebrated dome of Santa Maria del Fiore, built by Bruneleschi, is far superior in construction to the domes of Santa Sophia and San Marco. Bruneleschi first constructed the octagon tower which supports the dome. Each face of the tower is pierced with a circular window; the walls are 17 feet thick, and the cornice which terminates the tower is 175 feet from the ground. From this cornice rises the double dome. The external dome is 7 ft. 10 in. thick at the base. The internal dome, which is connected at the angles with the external dome, is 139 ft. in diameter and 133 ft. high from the top of the internal cornice of the tower to the eye of the lantern. This dome has eight angles, forming a species of Gothic vault, and was the first double dome with which we are acquainted. Some time after the dome was finished, several fractures were perceived in it, which were owing to settlements in the masonry; but the fractures were filled up, and no new signs of settlement have showed themselves Since. The first modern dome constructed in Rome was that of the Church of Our Lady of Loretto. It was commenced in 1507 by Antonio Sangallo. The dome, which is double, is circular on the plan. The internal dome is constructed on double consoles, instead of corbellings. The double consoles are crowned with a small cornice, forming an impost for eight arches, from the upper part of which springs the dome. On the top is a lantern light, which is not apparent externally. Up to this time domes had been constructed on walls and corbellings; but in St. Peter's at Rome a new plan was adopted. The dome of St. Peter's stands upon four piers, 61 ft. 11 in. . , and 30 ft. 10 in. thick, measured in a straight line with the arches. From the arches spring the corbellings, which are finished by an entablature. Upon this entablature is a plinth. The plinth is externally an octagon, and internally a circle. The external diameter of the octagon is 192 ft. 9 in., and the internal circle 134 ft. 84 in. ; the thinnest part of the wall, between the octagon and the circle, is 29 ft. 3 in. On the plinth is a circular stylobate, 28 ft. 64 in. thick. This thickness is divided into three parts by a circular passage, 5 ft. 10 in. wide : the two walls on each side of this passage are, respectively, the internal wall 14 ft. 7% in thick, and the external 8 ft. In the internal wall are other smaller passages, 2 ft. 10 in. wide, forming flights of steps communicating with the four spiral staircases formed in the thickness of the wall of the drum of the dome. Above the circular stylobate, which is 12 ft. 44 in. high, is placed the drum of the dome, which is 10 ft. 1% in. thick, measured to the inside line of the pilasters, winich decorate the interior of the dome. The pilasters themselves are 1:78 ft. thick in addition. The construction is formed of rubble and fragments of brick. The interior is lined with bricks stuccoed. Externally the work is faced with thin slabs of travertine stone. The drum is pierced with 16 windows, 9 ft. 33 in. wide and 17 ft. high. The walls are strengthened on the outside, between the windows, with 16 buttresses, constructed with solid masonry. These buttresses are 13 ft. 3 in. wide and 51 ft. 6 in. in height from the base to the top of the entablature. Each buttress is decorated and strengthened with half pilasters, and terminates with two coupled columns engaged, the diameter of which is 4 ft.: the order is Corinthian. hen the base of the dome had been built to the height of the entablature of the drum, Michel Angelo died; but some time before his death he had caused a wooden model to be made, with ample details, to which he added drawings and instructions. After his death Pirro Ligorio and Vignola were appointed the architects. Giacomo della Porta, the pupil of Vignola, followed his master as architect to the cathedral ; but though the designs of Michel Angelo were strictly followed, the dome itself was constructed under the pontificate of Sixtus W. Sixtus gave
Giacomo della Porta as a colleague Domenico Fontana, by whom the dome was .." On the constructions of Michel Angelo a circular attic was first formed, 19 ft. 2+ in. high and 9 ft. 7 in thick. This attic is strengthened externally by 16 projections, 2 ft. 11 in. deep and 6 ft. 4} in. wide, placed over the buttresses of the dome. On the attic rises the double dome, the internal diameter of which, at the base, is 138 ft. 5 in. The curve externally is an arc of a circle whose radius is 84 ft. 1-62 in. To the height of 27 ft. 8 in. from the attic the dome is solid. At the base the thickness is 9 ft. 7 in. ; and as the external dome is raised higher than the internal dome, the thickness is increased as the curve ascends, so that where the dome is divided the thickness is 11 ft. 4 in. The circular space which divides the two domes is 3 ft. 2+ in. wide; the internal dome is 6 ft. 4 in. thick; and the height from the attic to the opening of the lantern is 83 ft. 10 in. The diameter of the lantern is 24 ft. 10 in. The external dome is 2 ft. 104 in. thick where it separates itself from the internal dome; and it is strengthened externally by 16 projecting bands of the same thickness. The dome is pierced with three rows of small windows. As the curves of the dome are not concentric, the space between them becomes wider as it rises; so that at the opening of the lantern the space is 10 feet wide. These domes are joined together by 16 walls or spurs, diminishing in thickness as they ascend to the lantern ; at the base they are 8 ft. thick, and at the summit 3 ft. The base of the lantern is arched, and pierced with small windows. Above the two domes is a circular platform, surrounded with an iron gallery. In the centre rises the lantern, on a stylobate broken into 16 parts, forming projecting pedestals, above which are buttresses similar to the buttresses of the drum, decorated externally with coupled Ionic columns, 17% in. in diameter. The space between the buttresses is filled with arched openings, which give light to the lantern. The external diameter of the lantern is 39 ft.; the internal diameter 25 ft. 163 in. ; and the height from the platform to the top of the cross is 89 ft. 73 in. The whole height, from the external plinth of the dome to the cross, is 263 ft. The total height from the pavement is 437 ft. 5 in. The total height internally, to the top of the dome of the lantern, is 387 ft. Sixtus V. covered the external dome with lead, and the bands with bronze gilt. One hundred thousand large pieces of wood were used in making the centering of the domes, which was so admirably constructed, that it appeared suspended in the air. (See the drawings in the work by Fontana, on the construction of this dome.) This centering was more for the purpose of a scaffolding for the materials and workmen, than to sustain the weight of the double dome. During the construction of the dome it is believed that only two circles of iron were placed round the masonry, one of which was placed on the outside of the internal dome, at about 36 feet from its springing, and one foot above the division of the domes. The bands of iron of which this circle is composed are 3 in. wide by 13 in. thick. A similar circle is placed about the middle of the solid part of the dome, at about 17 feet 6 inches above the springing of the internal dome. Near the top of the internal dome there are several holes, at the bottom of which upright iron bars appear. These bars are said to be the connecting rods which keep together other circles of iron placed at different heights within the masonry, which are finally terminated by a circke round the eye of the dome. The domes were constructed with such haste, that sufficient time was not allowed to the work to form solid beds as it was carried up, in consequence of which a great number of vertical settlements took place, and the circle of iron round the internal dome was fractured. To obviate the danger arising from these settlements, six circles of iron were placed round the external dome at different heights, and the broken circle of the internal dome was repaired. The first circle was placed above the cornice of the external stylobate, or continuous plinth, on which the buttresses stand; the second circle was placed above the cornice of the buttresses, the third above the attic at the springing of the external dome, the fourth half way up the external dome, and the fifth under the base of the lantern. A sixth was shortly after placed at one foot below where the dome divides itself. The iron bands are flat, from 16 to 17 feet long, 3, inches wide, and 2% in. thick. At one end of the pieces of iron a hole is made; the other end is turned