This has been most strikingly shown in the application of mechanical engineering to several branches of iron manufacture.

There are few things which illustrate the giant power of machinery more entirely than the manufacture of armour-plates. A number of scientific men, and some of the Lords of the Admiralty, witnessed recently a great experiment with some new Rolling Mills belonging to John Brown of Sheffield. These rolls have a first foundation of no less than 60 tons of solid iron, resting on masonry carried far below the earth. The rollers themselves are 32 inches in diameter, and 8 feet wide, and are turned by an engine of 400-horse power. A powerful screw, applying its force through compound levers, allows the distance between the rollers to be adjusted to the fraction of an inch, so that the plate which on its first rolling, is forced through an interval of-for instance-12 inches apart is, on its next, wound through one of ten inches, next through one of 8 inches, and so on until the required thickness has been carefully and equally attained by compression through every part of the metal. When the enormous mass of iron to be rolled was first taken from the heating furnace, and brought to the rollers, it was found that they did not bite directly the mass came to them, and when they did, the engine was almost brought to a stand-still by the tremendous strain upon it; but at last the soft plate yielded, and the rollers wound it slowly in, squeezing out jets of melted iron, that shot about as the pile was compressed from 19 inches to 17 inches by the force of the rollers. From the time the mass had once passed through the mill, it was kept rolling backwards and forwards, the workmen sweeping from its face the scales of oxide that gathered fast upon it. Every time the plate was passed through, the rollers were squeezed closer and closer together, until at the end of a quarter of an hour from leaving the furnace, an almost melted mass, it was passed through the rolls for the last time, and came out a finished armour-plate, weighing 20 tons, 19 feet long, nearly 4 feet wide, and exactly 12 inches thick throughout from end to end.

Attention has been directed by Lieut-Colonel H. Clerk, R.A., to a matter of some engineering importance, "The Change of Form assumed by Wrought Iron and other Metals when Heated and then Cooled by partial Immersion in Water." The experiments recorded in a communication made by Colonel Clerk to the Royal Society originated in this way:

"A short time ago, when about to shoe a wheel with a hoop tire, to which it was necessary to give a bevel of about 3th of an inch, one of the workmen suggested that the bevel could be given by heating the tire red hot, and then immersing it one-half its depth in cold water. This was tried and found to answer perfectly, that portion of the tire which was out of the water being reduced in diameter." These experiments have an important bearing on many engineering problems; the general result appears to prove that metals heated to redness, and partially cooled, by having one portion only placed in cold water, contract about one inch above the water line, and that this is the same, whether the metal be immersed one-half or two thirds of its depth.

VIII. PHOTOGRAPHY.

By far the most important subject which has arisen in this branch during the last quarter, or, indeed, for many years past, is the alleged discovery of photographs taken half a century before the recognized birth of this art. An immense mass of evidence, direct and collateral, has been collected together in the most conscientious and energetic manner by Mr. Smith, Curator of the Patent Museum, and it certainly affords strong grounds for the presumption that no less than three, if not four, distinct classes of pictures, each by a different process, produced about the year 1790, are now in existence, there being the strongest circumstantial evidence that they are bona fide photographs. One is on a silver plate, pronounced by leading members of the Photographic Society to be an undoubted photograph from nature, the subject being Mr. Boulton's house, which was pulled down in 1791; the picture was found amongst papers in Mr. Boulton's library, which had not been disturbed during the present century. There are also two pictures-one of them undeniably a photograph-which were found by Miss Meteyard amongst papers supplied to her for the purpose of writing a life of Wedgwood, the great potter; and from documents of that date they are said to have been produced by the younger Wedgwood, reference being made to a lens, camera, and chemicals. There is also the hearsay evidence of an old retainer of the Boulton family, lately dead, of the silver picture of Mr. Boulton's house having been taken by placing a camera on the lawn; and there was a society called the 'Lunar Society,' the members of which were said to produce pictures by using a dark room, throwing the images on to a table, and fixing them by some chemical. The whole subject has recently been brought before the Photographic Society, and, on a careful analysis of the evidence, there is the very strongest presumption, short of absolute certainty, that this important discovery was made, and then suffered to die out. Only a few links in the chain are wanting to establish the actual proof, and from the intense excitement the subject has now occasioned, there is little doubt that it will be sifted to the bottom.

The measurement of the chemical action of light has lately received considerable attention. Dr. Phipson* has published a process which appears to promise very good results; it is based upon the fact, that a solution of sulphate of molybdic acid is reduced by the action of light to a lower state of oxidation; and by measuring this amount of reduction by chemical means, a correct estimate of the amount of actinism used up in the operation is obtained. The measurement is done with a standard solution of permanganate of potash; and Dr. Phipson states that his observations have disclosed the fact, that the amount of actinism during the day varies considerably, describing curves, which are not only irregular, but sometimes present sudden deflections of considerable extent. This phenomenon has been noticed before. During the last summer many correspondents of the Photographic *Chemical News,' vol. viii. p. 135.

News' have stated that, on certain days during particular hours, there seemed to be an almost total absence of actinic force. In some instances five and six times the ordinary exposure were given with very imperfect results; and in other instances twenty times were tried with no effect. No particular atmospheric influence could be detected at work; and on subsequent days, apparently identical in light and clearness, photographic operations were conducted with their usual celerity. The cause of this great variation appears to have some connection with the dryness of the atmosphere, the days on which the absence of actinism was most marked having been intensely hot and free from humidity. It is much to be desired that a simple system of actinometry should come into general use. The processes of Draper, Niépce de St. Victor, Bunsen and Roscoe, Herschel, Phipson, and others, are very useful, but rather too tedious for general use. What we want is some method of reading off the amount of actinism as simply as we read off the amount of heat with the thermometer.

A most elaborate series of researches on the behaviour of chloride, bromide, and iodide of silver in the light, and on the theory of photography, has recently been published by M. H. Vogel. The researches have extended over three years, and are of the most exhaustive character. We have only space to give some of the bare results which he has obtained, and must refer our readers for further particulars to the original memoir. The author considers that the action of light upon chloride and bromide of silver is first the production of a subchloride and subbromide, with liberation of chlorine and bromine, but that the iodide of silver undergoes no chemical change whatever. The action of acids and various saline solutions, especially nitrate of silver, has been studied very carefully, and some of the results are of considerable value. The effect of developing agents has been likewise examined, and the whole memoir constitutes one of the most important contributions to the science of photography ever published.

A valuable improvement has been inaugurated in the manufacture of lenses for photographic purposes. By the ordinary method of grinding and polishing, the surface is not left in a state of perfection anything approaching that required for astronomical glasses. For the usual photographic processes this surface is quite good enough, although, when carefully examined with a powerful glass, it will be seen covered with irregularities, the remains of the last stages of the grinding process. To attain greater perfection entirely different means have to be employed, and the costly nature of this operation is one reason why telescopic lenses are so valuable. For some purposes, however, in which it is absolutely necessary to get perfect delineation, as in the copying maps, &c., a lens ground in the ordinary way would be inapplicable, and perfection must not be hoped for unless the lens possesses a perfectly continuous spherical surface with the highest possible polish. Mr. Osborn, the photographer to the Melbourne Government, who is engaged in copying maps for the Melbourne Survey Office, has just ordered a lens from Mr. Dallmeyer, the cost of which is to be * Poggendorf's 'Annalen,' 1863, p. 497.

2501. It will be a triple achromatic, and the glasses will probably require months for their completion, during the whole of which time the grinding and polishing machinery will have to be moving under the personal superintendence of one of the first practical opticians in England. The experiment is necessarily a costly one, and photographers are naturally anxious to see if the result compensates for the enormous additional expense. The Melbourne Government deserve the thanks of all photographers for the spirit of enterprise they have shown in the matter.

From time to time rock crystal lenses have been recommended on account of their superior transparency to the chemical rays of light. Mr. Grubb has put this theory to the test of experience, and finds that the difference is not so great as has been imagined; for instance, a compound lens of the ordinary make transmits 87 for every 100 rays which the rock crystal allows to pass. It is therefore only 13 per cent. worse, whilst in flatness of field and achromatism, the glass lens is much superior.

M. Gaudin suggests that lenses should be made from fused rock crystal. The manufacture of these is simply a question of expense, and they might possibly be achromatized by the employment of other suitably transparent minerals.

A new fixing agent, sulpho-cyanide of ammonium, is likely ere long to supersede hyposulphite of soda. The advantages claimed are, permanence of the print, and great facility in the washing operations; but, on the other hand, the expense is likely to be an objection. A little time ago, the new agent cost 48. an ounce; there are rumours that it can now be procured in Paris for 18. 14d. per lb., although we have been quite unable to obtain any at this price, and Mr. Spence, the manufacturing chemist of Manchester, has just erected large apparatus, by means of which he hopes to supply the sulpho-cyanide at even a less price. We may therefore reasonably anticipate that sulphur toning, yellow whites, and fading positives, will soon have gone the way of the Dodo and Megatherium.

Celestial photography is making great strides on the other side of the Atlantic. Dr. Henry Draper has just completed a large reflecting telescope, 16 inches in aperture, and 13 feet focus, which he intends to devote to this branch of science. The mirror is of glass, covered at Sir John Herschel's suggestion, with a film of precipitated silver. It is sustained in a walnut tube, hooked with brass, and specially mounted to avoid tremor. When in use the instrument is allowed to be at rest, clockwork being used only to drive the sensitive plate. By this means, only 1 oz. instead of half a ton, is moved. A photographic laboratory is attached to the observatory, and the apparatus is arranged to take photographs of the moon as large as 3 feet in diameter, being on a scale of less than 50 miles to the inch. From the reputation which Dr. Draper has already earned as a photographer and physical philosopher, we are justified in expecting that celestial photography will advance rapidly in his hands.

LIGHT. The cause of the scintillations of stars has long been a puzzle, not only to children, but to philosophers. Many explanations have been given, but none are quite satisfactory. Mr. A. Claudet * has thrown some new light upon this subject, by an instrument which he has devised, called the chromatoscope. He attributes the beautiful sparkling, with changing colours, exhibited by certain stars on a clear night, to the evolution in different degrees of swiftness of the various coloured rays they emit. These rays are supposed to divide during their long and rapid course through space, and we see them following each other in quick succession, but so rapidly that, although we see distinctly the various colours, we cannot judge of the separate lengths of their duration. Mr. Claudet's instrument consists of a reflecting telescope, part of which is caused to rotate eccentrically in such a manner, that instead of a point a ring-like image of the star is seen. The rapidity of rotation is adjusted so that each separate colour given by the star is drawn out into a large segment of the ring, and in that manner the light from the star can be analysed as in a spectro

scope.

In observing the rays of sunlight through a powerful spectroscope many additional lines are visible when the sun is near the horizon. These are called telluric rays, as they have been shown to owe their existence to some components of the earth's atmosphere. Father Secchi, the Roman Astronomer, considers that aqueous vapour in the atmosphere is the principal cause of these telluric rays, and this opinion has been generally adopted by physicists: but M. Volpicelli † now describes experiments to prove that these rays are independent of the presence or absence of aqueous vapour in the atmosphere. In our opinion his experiments are scarcely conclusive; for it is quite reasonable to suppose that the passage of light through 100 miles of atmosphere might produce effects which could not be imitated in a laboratory experiment.

The determination of the refracting power of various transparent liquids and solids, a matter of considerable practical importance, is usually effected by reference to certain well-known lines in the solar spectrum. It would be much easier to have recourse to the bright spectral lines of coloured flames, which are obtainable with ease at any time, whereas the employment of Fraunhofer's lines is dependent on the weather. For accurate experiments it is necessary to know the length of the waves for the differently coloured rays, and this information has been supplied by Dr. J. Müller, by means of one of Nobert's well-divided glass screens. His results show that the length of wave * 'Phil. Mag.' No. 175.

+ Cosmos,' vol. xxiii. p. 430.

Poggendorf's 'Annalen,' vol, cxviii. p. 641.