Page images
PDF

Synoptical Table of the Genera, according to De Blainville.
so - - Spatangus.
Subterminal To { Ananchites,
Nucleolites.
I Echinoclypeus.
without teeth . Já.”
Fibularia.
Subcentral Ethnoneus.
Mouth & Echinocyamus.
Laganus.
Armed with teeth & Clypeaster.
Echinodiscus.
Scutella.
Infra-lateral . {Galerites.
o ; Echinometra.
en Central . {#
Cidaris.
Sub-Family. 1.
Eaccentrostomata.
Genera. Spatangus.

Body oval, more or less elongated, heart-shaped, wider before than behind, with a furrow more or less profound at the anterior extremity. Shell delicate, of little solidity, composed of large polygonal plates, not many in number. Spines short, flat, sessile and scattered. Ambulacra incomplete, only four in number. Buccal notch more or less anterior, transverse, bilabiated, circumscribing a mouth without teeth. Vent terminal, and rather above than below the border. Genital pores four in number, disposed in two pairs. The species are very numerous, and are subdivided by De Blainville and others into sections according to their shape, &c. The following is De Blainville's method. Co. Species whose ambulacra are not petaloid, and form scarcely but two lines, a little broken or bent at their internal side, and which have a rather deep anterior furrow, and the mouth not much in front. Example, Spatangus arcuarius. De Blainville observes that Mr. Gray places Spatangus Atropos in this section; but the former thinks that it sensibly differs from those classed under it, and places it in the following section.

[merged small][merged small][ocr errors][merged small][merged small]

Spatangus cordalus, Ananchites cordatus, Lam.,) with ambulacra reaching to the border. Geographical distribution.--In almost all seas, including our own. Numerous in the Mediterranean. Habits.-Not known, but they seem to live constantly burrowed in the sand. Food-De Blainville supposes that the Spatangi are nourished with the animal matters which are mingled with the sand; for their intestinal canal, which is thin as a spider's web, was always found by him full of fine sand.

Fossil SPATANGI.

The species are numerous in the chalk and cretaceous group, and occur in the oolitic group. Ananchites. (Fossil only.) Body oval in its longer diameter (from before backwards), rounded and a little wider, but without a furrow, anteriorly, subcarinated posteriorly, conical, elevated at its summit, which is mesial, entirely flat below, covered with a very few small scattered tubercles. Ambulacra, to the number 9f five, rather large, divergent, comprised between double lines of pores but little approximated, and scarcely overpassing the borders. Mouth and vent subterminal and inferior. De Blainville subdivides this genus into two sections: the first, with the ambulacra prolonged up to the borders (Ananchites, Lam.), example, Ananchites ovatus; the second, with the ambulacra prolonged up to the mouth (Echinocorys, Leske, Gray; Galea, Galeola, Klein), example, Ananchites pustulosus, Echinocorytes pustulosus, Leske. M. Defrance enumerates 12 species: to these are to be added M. Risso's three species, A. carinatus, A. rotundatus, and A. stella, if they be distinct. De Blainville observes that Lamarck's Ananchites ellipticus most probably does not belong to this division, and that his A. Cor avium belongs to the same division as the Violet Spatangus Goldfuss has described some new species. Sub-Fumily II. Paracentrastomata Edentata. Genera. Nucleolites. (Fossil only.) Echinobrissus of Breyn and Gray, adding the Cassiduli. Body oval or heart-shaped, wider and with a large furrow behind, rather convex, the summit subcentral and moderately elevated above, somewhat concave below; covered with small, equal and scattered tubercles. Ambulacra, to the number of five, subpetaloid, open at the extremity, dorsal and marginal, and continued by as many furrows up to the mouth, which is inferior, subcentral, and anterior. P'ent subcentral, above, in the furrow. Genital pores to the number of four. Example, Nucleolites depressus, Spatangus depressus, Leske, Klein; “s. lobatus, Fleming. Locality, &c.—The species are tolerably numerous and are frequent in the chalk, but are also found in the beds anterior and posterior to it. Echinoclypeus. (Fossil only.) Body depressed or conical, circular or inclining to oval, with a furrow behind, convex and with a subcentral summit above, rather excavated below, formed of distinct plates and covered with very small equal tubercles. Ambulacra to the number of five, dorso-marginal, subpetaloid; the double rows of pores united by a transverse furrow. Mouth subcentral, a little more anterior, pentagonal, with five converging, ambulacriform furrows. P'ent entirely above, behind the summit, and at the origin of the posterior furrow. Genital pores to the number of four. De Blainville remarks that this generic section, established by Klein under the name of Clypeus, has been confounded by Lamarck with his Galerites, which belong to an entirely different division of the Echinidae; and he observes that they might much better be confounded with the Nucleolites, after the arrangement of Defrance. He adds that he should not be surprised if the Cassidulus scutella belonged to this division. Echinolampas, Gray. (Echimanthus? Leske.) Body oval or circular, depressed, subconvex above, rather concave below, rounded and widened forward, rather narrowed towards the anal extremity, composed of great polygonal plates and covered with spines, probably very shal; Ambulacra, to the number of five, subpetaliforim, not closed at their extremity, and nearly o border. Mouth round, subcentral, and nevertheless, a ittle anterior Vent entirely marginal, terminal. Genita' pores four only in number Example, Echinolampas oriental on). 2 L 2

The form occurs fossil; see, for instance, Trans. Geol. Soc. (Second Series) i., tab. 3, fig. 3, 4, 5. (Echinomăus Lampas.) Cassidulus.

Body oval, more or less depressed, composed of indistinct lates and covered with small spines. Ambulacra five, orsal, rarely marginal. Mouth below, submedian, in a stelliform notch. Vent postero-dorsal, or above the border. Genital pores four. De Blainville subdivides this genus into the following sections:at. Species whose ambulacra form a dorsal star, and whose mouth is at the bottom of a stelliform impression. Example, Cassidulus Lapis Cancri. 3.

Species whose ambulacra are prolonged to the border and not closed. Example, Cassidulus Australis.

Y. Species whose ambulacra are not known to De Blainville. Example, Cassidulus scutella. De Blainville observes that this genus (Lamarck's) is evidently artificial; for that the position of the vent cannot furnish any character of much importance. He remarks that there is but one recent species; the others, to the number of nine, according to Defrance, are fossil, from the beds anterior to the chalk, and with some little doubt, from more recent formations. Goldfuss unites the genus with Nucleolites. Fibularia.

Body globular, but rather higher than it is wide, ribbed, as it were, with about twenty ribs, formed probably by so many ranks of polygonal scales, and covered with very fine spines. Ambulaera five, very short, and not shut at the extremity. Mouth round, subcentral. Vent inferior and much approximated to the mouth. Genital pores unknown. Example, Fibularia craniolaris.

De Blainville observes that this genus was established by Van Phelsum and by Leske, under the denomination of Echinocyamus, adopted by Mr. Gray. De Blainville only leaves under it F. craniolaris and the seven or eight but little distinguished or indistinct species which Van Phelsum established, and probably the C. trigona of Lamarck, but he says that he has seen none of them; and he adds that, in the genus as defined by him, only living species have yet been found.

Echinoneus.

Body rounded or oval, generally excavated below, composed of plates often distinct and covered with small spines. Ambulacra five, large, complete, radiating from the dorsal centre to the mouth, and formed by ambulacral lines, which are very close and impressed. Mouth central or subcentral, without teeth, and pierced in a subtriangular hole of the shell. Vent towards the border below or even above, in a longitudinal and subsymmetrical hole of the shell. Genital pores four.

De Blainville subdivides the genus into the following sections:—

at.

Oval species, with the anal hole longitudinal and below. Example, Echinoneus minor.

Circular species, with the vent below and round. (Discoidea, Gray.) Example, Echinoneus subuculus. Y. Oval species, with the vent entirely marginal, and the genital pores to the number of seven 2 Example, Echinoneus ovalis. 3. Circular species, which are depressed and have a margino-dorsal, nonsymmetrical anal opening. Example, Echinoneus cassidularis. De Blainville observes that no Echinoneus with the anal opening below is known in a fossil state; so that in the genus, as defined by Lamarck, there are no fossil species according to Defrance; but that in his (De Blainville's) method of arrangement there are many; and he remarks that Goldfuss figures four species from the chalk, but he ds a query whether they belong to this genus.

Sub-Family III. Paracentrostomata Dentata.

Mouth subcentral, in a regular notch of the shell, and

provided with teeth.
Genera. Echinocyamus.

Body depressed, oval, wider behind than before, a little excavated below, covered with rounded tubercles pierced at the summit and rather large in proportion, supported internally by five double inferior ribs, terminating round the buccal notch by as many simple apophyses. Am dorsal, not marginal, completely open at the extremity, a little enlarged, and forming a sort of cross with dilated branches. Buccal opening subcentral, regular, armed with five teeth as in Clypeaster. Vent below, between the mouth and the border. Genital pores four. Example, Echinocyamus minutus.

De Blainville states that he characterized this genus from a considerable number of individuals of a very small species found in the intestines of a turbot, and which occurs in great quantity in the sand of the coasts of the English Channel, according to Pallas, both on the French and English shores. He adds that, very probably, it is the Fibularia ovulum of Lamarck; and that, without doubt, Fibularia Tarentina belongs to this genus, as well as Echinoneus Placenta of Goldfuss.

Lagana, Gray. (Echinodiscus, Von Phelsum, Leske.)

Body depressed, circular or oval lengthwise, a little convex above, concave below, with an entire disk and borders, composed of plates but little distinct and covered with scattered spines. Ambulacra five, regular, petaloid, shut, or nearly so at the extremity, with the pores of each side united by a furrow. Mouth median in the middle of a hole, with converging furrows and furnished with teeth. P'ent inferior, pierced in a regular hole, situated between the mouth and the border. Genital pores five. The genus is thus sub-divided by De Blainville:

01. Circular species. Example, Lagana orbicularis.

8. Oval species. Example, Lagana ovalis.

Y. Polygonal species. Example, Lagana decagona. The genus approximates to Clypeaster, under which Lamarck arranges the species.

Clypeaster.

Body much depressed, rounded and rather thick on the borders, sometimes incompletely orbicular or radiated, enlarged towards the anal extremity, composed of large and unequal plates, covered with very small, equal, scattered spines supported on very small tubercles pierced with a pore. Ambulacra constantly five in number, dorsal, petaloid, the two rows of pores of each branch united by a furrow. Mouth central or sub-central, at the bottom of a sort of tunnel, formed by five grooves and armed with five teeth. Pent terminal and marginal. Genital pores to the number of five.

Living species few. Localities, the seas of warm coun

tries—in Asia and America.

Example, Clypeaster rosaceus.

Fossil species more numerous and generally from the tertiary beds. Defrance enumerates eleven, , Goldfuss figures ten new ones; but De Blainville adds a query whether they are all of this genus.

De Blainville states that this division of Echinidans was established by Breyn under the name of Echinanthus, which Mr. Gray has retained, and under that of Echinorodon by Van Phelsum.

Echinodiscus.

Body rounded, depressed, sub-quinquelobated (the posterior lobe a little notched in the median line), rather conical above, concave below, composed of plates in twenty rows, placed two and two. The ambulacraires narrower and covered with very small, fine, close-set spines. Ambulacra to the number of five, diverging by the complete separation of each double line of pores. Mouth median, round, towards which converge five straight and stelliform furrows. Went marginal. Genital pores to the number of four. Example, Echinus Parma.

Locality ofthe species.—De Blainville observes that it was

[blocks in formation]

Localities.—The living species whose habitat is known are foreign, and the South Seas appear to be their principal locality. Nevertheless, as De Blainville observes, we ought to remember that Defrance, in the description of a fossil species, Scutella Hispana, says, that it bears great resemblance to a species that lives in the English Channel and which is found on the coasts in the department of Calvados. De Blainville adds, that he has not seen this species, and that it is the first time we find it stated that a Scutella exists in our seas. None of the English, Italian, or French authors whom he consulted mention it.

[merged small][merged small][merged small][merged small][ocr errors][merged small]

£3. Species with five ambulacra. Example, Galerites vulgaris Y. Species with six ambulacra. Example, Galerites serJasciatus.

The genus is often found silicified and in casts. The greater portion belong to the chalk and a small number to the beds anterior to the chalk. None have as yet been found in the more recent strata.

Echinometra. (Gray.)

Body thick, solid, transversely oval, a little depressed, convex, with the summit (which is median) flat above and arched below, covered with mamillated tubercles of two sorts and bearing diversiform, but always strong and large spines. Ambulacra five, enlarging themselves below. Buccal opening of the shell large, transverse, with very powerful auricles on its internal circumference. Five sharp teeth at the mouth, with a complicated apparatus, as in Echinus. P'ent medio-superal or opposed to the mouth. Genital pores to the number of five. Example, Echinometra atrata.

Localities.—The seas of warm climates. those of England and France.

No fossil species known.

Unknown in

Echinus.

Body in general very regularly circular or sub-polygonal, sometimes slightly transverse, composed of twenty radiated rows, alternately unequal, of polygonal plates bristled with diversiform spines of two kinds, and supported on imperforate mamillated tubercles. Ambulacra constantly to the number of five and complete. Mouth central, armed with five pointed teeth, supported upon a very complicated internal apparatus. P'ent median, superior, or exactly opposite to the mouth, Genital pores to the number of five.

Food—The food is generally believed to consist of mollusks and crustaceans. Tiedemann found in E. Saaratilis small univalve and bivalve shells entire among the excrements, as well as fragments of larger ones. Bosc is said to have witnessed an Echinus in the act of seizing and devouring a small crustacean. Dr. Sharpey usually %. in the intestine of E. esculentus small morsels of sea-weed, for the most part encrusted with flustra; and he says that the excrements, which are in the form of small round pellets about the size of peppercorns, consist chiefly of sandy matter with fragments of shells. But he adds that it would be difficult to say whether these are the remains of digested mollusca or merely a portion of the usual testaceous debris so abundant in sand and mud.

[ocr errors][merged small]

3. Regular species, more or less convex, but, for the rest, diversiform; area sub-equal, bordered by a double series of pores, forming at the exterior, denticulations more or less marked and each with three pairs of holes. De Blainville subdivides this section into three, with still further subdivisions depending on the non-fissured or more or less fissured angles of the buccal opening of the shell, and other variations. He states that he has been able to study a great number of living species, and though many have been only known to him by means of the shell, he has been able, he says, to find constant specific characters: 1st, in the proportion of the ambulacral and anambulacral area ; 2nd, in the number of lines of double pores which limit the ambulacra; 3rd, in the number of those double pores which form the festoons of these lines; 4th, in the form of the auricles, serving for the insertion of the muscles of the dental apparatus; 5th, in the disposition of the border of the buccal orifice. He states as a result, that though he has indicated nearly double the number of species pointed out by Lamarck, they are much more easily recognized. - Localities, Habits, &c.—The form is widely diffused, and

[merged small][merged small][merged small][merged small][merged small][ocr errors]

Orbicular species, very depressed; interambulacral area equalling tho half of the others, and bordered by straight and very large ambulacra. (Astropyga, Gray.) Example, Cidaris radiata. Localities.—Seas of the southern hemisphere. Two species already known in the seas of Britain and France, one on the coasts of Scotland, rare; the other very common in the Mediterranean. FossiL SPECIEs. Cidaris occurs in a fossil state in the chalk and the an terior beds. Defrance mentions three, but hardly charac. terizes them. Risso adds two new ones; Fleming four; and Goldfuss has figured and characterized nineteen. Mr. Gray (Proceedings of the Zoological Society, 1835) divides the genus Echinus, as restricted by Lamarck and modern authors, into what he considers four natural genera, adapted to facilitate the distinction of the species of this extensive group. He regards this distinction as of the more importance, inasmuch as some of the characters which had been used for this purpose, such as the number of the tesserae and of the pores in the ambulacra, have been found to be inconstant; the number of these increasing, as they are now known to do, with the age of the specimens. The following is Mr. Gray's subdivision:-Genus 1. Arbacia. This corresponds with section A of M. de Blainville. Example, Arbacia pustulosa (Echinus pustulosus, Lam.) Genus 2. Salenia, only known in a fossil state, and hitherto confounded with Cidaris; but its tubercles are not pierced. Example, Salenia scutiger (Cidaris scutiger, Munst.) Genus 3. Echinus containing sections B*, C, E, and G of De Blainville. Mr. Gray divides it into two sections:— 1. The species with narrower ambulacra and with the pores moderate and approximated, which is subdivided into those with a subintegral mouth (type, Echinus esculentus) and those with the mouth deeply incised. (Example, Echinus ercavatus, Lam.) 2. The species with wide ambulacra; the pores separated by small tubercles; the mouth five-in.. Example, Echinus ventricosus, Lam. Genus 4. Echinometra, containing sections B", D, and F of De Blainville, as well as the Echinometra of that author. In this genus Mr. Gray observes the ambulacral plates may be considered as .# composed of five or more doublypierced pieces, which form an arched line round the outer edge of the tessera, with a single pair of pores at its lower inner angle. , Mr. Gray stated that he had formerly separated from the Echini some of the species of this genus, which are peculiar for their oblong form, and that the enus so proposed by him had been adopted by M. de lainville; but a much more extended examination had convinced Mr. Gray that individuals of the same species vary from roundish to oblong; and therefore, having observed many round species agreeing with the oblong ones

in the peculiar character of the ambulacra, he has united them to the former under the same name. Mr. Gray remarked, as throwing doubt on the bilaterality of the Echinidae attempted to be established by M. Agassiz, that the spongy ovarial plates which that gentleman regarded as the mark of the hinder part of the Echinidae, is alwavs placed on one side or the other of the longer axis of the oblong species. See also Mr. Gray's paper on the genera of these animals in the “Annals of Philosophy;' and Dr. Sharpey's article ‘Echinodermata' in the “Cyclopædia of Anatomy and Physiology.’ ECHINOBRI'SSUS. [EchiniDAE, p. 259.] ECHINOCACTUS, a genus of Cactaceous plants, with the stem of an ovate or spheroidal form, the sides being divided into many ribs, upon whose projecting angles are stationed at short intervals little spiny stars, which are the rudiments of leaves, and from whose centre the flowers appear. The latter consist of numerous sepals collected into a tube, an equally large number of petals, numerous stamens, and a filiform style divided into many lobes at the point. The species are very remarkable for the singular forms of their stems, and for the curious manner in which their spines are arranged. They are often moreover conspicuous for the beauty of their large flowers. The genus is extremely near Cereus, from which, according to De Candolle, it only dissers in having the sepals and petals distinct from each other, not united into a tube. But as Cereus triangularis has its sepals distinct, and all the Echinocacti have more or less of a tube, we consider it better to limit the latter to such species as have a depressed or spheroidal form. With such a limitation the Echinocactus Eyriesü, one of the most beautiful of plants, will really belong to the genus Echinocactus, of which it has all the habit; otherwise it would be a Cereus, to which its stems bear but little resemblance. Most of the species are naÉ. | Mexico and the West Indies. A few are found in Iolzll.

Echinocactus Eyriesú. ECHINOCIDARIS, p. 261. ECHINOCLY/PEU.S. [EcHINIDAE, p. 259.] ECHINOCONUS. [EchiniD.E., p. 261.] ECHINO'CORYS. [EcHINIDAE, p. 259.] ECHINOCY'AMU.S. [EcHINIDAE, p. 260.] ECHINODE/RMATA. Lamarck made his Radianres Echinodermes consist of three sections. 1st, the Stellirideans (star-fishes), including Comatula, Euryale, Ophiura, and Asterias; 2nd, the Echinidae; and 3rd, the Fistulidae, comprehending Actinia, Holothuria, Fistularia, Priapulus, and Sipunculus. Cuvier's Echinodermes form his first class of zoophytes, and this class is divided into two orders, viz., 1st, the Pedicillated Echinoderms, containing the great genus Asterias and its subgenera the Encrimites, the Echinidae, and Holothuria; and 2nd, the Footless Echinoderms, consisting of Molpadia, Minyas, Priapulus, the Lithoderms, Sipunculus, Bonellia, and Thalassema, with its subgenera Echiurus and Sternaspis. De Blainville's Echinodermata are placed as his first class of Actinozoa, and are divided into three orders: 1st, Holothuridea; 2nd, Echanidea [Echi NIDAE]; 3rd, Stelleridea, embracing the Encrinites as well as the Free Starfishes, &c. The Echinodermata belong to the Cycloneurose subkingdom. ECHINODISCUS. [EchiniDAE, p. 260.] ECHINOLAMPAS. [Echinidae, p. 259.] ECHINOMETRA. [Echinidie, p. 261.] ECHI’NONEU.S. [Echinidae, p. 260.] ECHINO'PORA. . [MADREPHYLliq:A.] ECHINORODON. [EcHINIDAE, p. 260.] ECHI'NUS. [EcHINIDAE, p. 261. ECHITES, a genus of twining Apocynaceous plants inhabiting tropical countries. They have handsome yellow or white corollas, and are moreover remarkable for the singular fruit, which consists of two divaricating woody podlike follicles containing a large number of silky seeds. They are dangerous lactescent plants of no known use. E'CHIUM, an irregular-flowered genus of Boraginaceous plants, with handsome campanulate corollas. Echium vulgare is the most striking of our wild herbaceous plants; many species found at the Cape of Good Hope are shrubs. ECHO. (#x0, ixoc, sound). When sonorous undulations are propagated from any origin through the elastic medium of the air, the spherical wavelike surface then generated conveys the sound through the circumjacent space, and moves from its origin and centre with a velocity of about 1125 feet in a second, at the ordinary atmospheric pressure and temperature; for the velocity of undulations propagated through elastic media depends only on their indices of elasticity and not on their intensity. {{j

[graphic]
[ocr errors][ocr errors][ocr errors][ocr errors]

Suppose the point O to be the origin of a sound which in its progress encounters a plane obstacle NM; if this plane be sufficiently extended, a point M may be easily found which the sound will have just reached at the end of a given time. The waves which have previously reached the nearer points A,B,C, being precluded from advancing, are there reflected, that is, new spherical undulations a'ab, b/bc, c'cd are generated from A,B,C as centres, and their radii at the moment we have spoken of are respectively Ab =OM–OA, Bc =OM–OB, Cd=OM–Oc, and it is easily seen that all these spherical surfaces originating from A up to M and existing simultaneously, may be exactly enveloped by a single por. tion of a spherical surface of which the centre is placed in a position R corresponding to O in respect to its distance from NM, but at the opposite side of the obstacle; this spherical surface, of which the radius is RM, is the true returning wave at that moment, and being impressed on the audito organs, so as to be distinguished from the original sound, is called the echo.

[ocr errors][ocr errors][ocr errors][merged small]

the space of a comical frustum aa Cc, the vertex of the cone R being situated symmetrically with O at the opposite side of AC. In order that a person may hear the echo of his own sounds, it is therefore necessary that his situation may be at a point O' in a perpendicular to AC; and that a second person may hear the echo of the voice of another at O, he must be situated in the frustum aa Cc, so that the angles of incidence and reflection of the sound which reaches his ear may be equal; in both cases the distance from AC must be sufficiently great to distinguish between the original and the reflected sound. Hence it follows, that wherever a person is situated, the echo of a single sound necessarily follows and cannot precede the original sound, for the two sides OA, Aa are greater than the third Oa through which the direct sound is propagated, and the velocities are in both cases alike. However, the echo of a continued sound or note may be heard in the inverse order of time to that in which it was generated, provided the origin of the sound moves more rapidly towards the hearer than the rate at which sound travels. Thus a flash of lightning moving towards a person will produce a roll of thunder which, echoed by clouds, will be heard as it were backwards; but if the direction of the flash be such that the points of its current are nearly equidistant from the auditor, an instantaneous and intensely loud clap will be substituted for a continued roll. The murmuring sound produced by the discharge of great guns is the succession of echoes from the particles of vapour floating in the atmosphere, and when the discharge is effected j. a dense cloud, the echoes are stronger and better reflected, and a noise resembling a thunder-roll may then be heard. The whizzing of a bullet is attributed to its impinging in a state of rapid rotation on particles of vapour. The time intervening between the primitive sound and its echo has sometimes been employed in determining the distance from the observer to the reflecting object, allowing 571 feet for each intermediate second of time; but like all methods dependent more on individual judgment than mechanical measurement, this process must be liable to considerable irregularities. When several objects reflect sound, the number of echoes is greatly multiplied, not only from the primary echoes of each, but also from secondary and tertiary echoes by second and third reflections of returning waves against the reverberatory obstacles: each re-echo consists of only portions or frusta of the preceding; their intensities therefore diminish, and they gradually die away upon the ear, in the same manner that the images become obscure and by degrees imperceptible in consequence of the diminution of light when we look between two opposite and parallel plane mirrors. The first echo heard in such circumstances is by no means necessarily the loudest. Taking any ellipse of which one focus is the origin of the sound and the other the place of the auditor, it is a well-known property of this curve that right lines drawn from the foci to any point in it make equal angles with the tangent at that point. Con. ceive now this ellipse to rotate round the line joining the foci so as to form a prolate spheroid, then sound emanating from one focus and reflected by a portion of the surface will be directed after reflection to the other, and its intensity will depend on the solid angle subtended at the focus by the o body. Each echoing body may be conceived as a portion of such a spheroidical surface, taking a great axis major to comprehend the more distant bodies; and since the sum of the solid angles subtended by the more distant reflectors may be greater than those given by the nearer, the echo produced by them, though not reaching the ear as soon as that of the nearer, may, under such circumstances, be louder, bearing in mind in our estimate that this intensity has a source of diminution in the increase of distance. This case frequently occurs in places encompassed by chains of mountains, as the Killarney and Welsh lakes, &c. When the succession of echoes from several bodies is sufficiently rapid, a continued sound or note may be produced, though the original sound was merely momentary; and when not sufficiently rapid for this purpose, a clamorous noise is produced, and hence Echo with her thousand tongues and babbling propensities has furnished matter for }. imagination from Ovid to Shakspeare. As a single a may be converted into an imitation of a stunning laugh, the romantic and : regions, inhabited by the Scandinavian races materially assisted their untutored ima

« EelmineJätka »