An instrument so far surpassing all formerly known in the sensibility of its indications, has, in the hands of its distinguished inventor, led to a series of results equally new and remarkable. A brief summary of some of the chief of them is as follows:

Radiant heat passes directly, in greater or less quantity, through certain kinds of solid and liquid bodies. This class of bodies does not precisely include those which are transparent, since some which are opaque, or very little transparent, are the most "diathermanous*," that is, transparent, as it were, to heat. This term is one which M. Melloni has introduced as descriptive of the characteristic in question, and which we shall continue to

use.

He concludes, in general, that there exist different species of heating rays; and that all these different kinds are emitted simultaneously from luminous hot bodies; though in different proportions from different sources, certain of them are entirely wanting in non-luminous hot bodies.

Rock-salt, cut into plates, and successively exposed to the radiations from different sources, transmits in all cases the same proportion of heat. Plates of any other diathermanous substance, under the same circumstances, transmit a less proportion of heat as the temperature of the source is less elevated; but the differences between one substance and another in this respect, diminish as the plate is of less thickness; whence it follows (according to M. Melloni) that the calorific rays from different sources are intercepted in a greater or less degree, not at the surface, or in virtue of an absorbing power which varies with the intensity, but in the interior of the plate, by a peculiar absorptive force, which is analagous to that of coloured media for particular rays of light.

M. Melloni advances several theoretical views in support of this analogy. He remarks, in general, that there is but one substance (viz. rock-salt) of all he has tried, which is transparent and uncoloured, and acts really in the same manner both on the rays of light and of heat. All others, though they allow all rays of light to pass indifferently, yet absorb certain rays of heat and transmit others. We thus recognise, by means of these bodies, a true distinction in heat corresponding to that of colour in light.

The colouring matter of transparent media always diminishes more or less their diathermanous properties, but gives them no peculiar property of stopping, by preference, any particular species of heating rays. It operates upon the transmission of radiant heat, as brown colouring matter (a smoked glass for instance) does upon light; that is, has only a general diminishing power on the intensity. There seems to be, however, an exception in regard to certain glasses coloured with green and with opaque black; but these two kinds of colouring matter only appear to act in modifying the quality of the diathermanous property.

Glass intercepts wholly certain species of heating rays, including all those which come from bodies below luminosity; hence, in this last case, no refraction by prisms or lenses has ever been effected for such rays of heat. With rock-salt, however, the case is different; and it is unquestionably the most singular and important of the facts elicited by Melloni, that simple heat is not merely transmitted through rock-salt, but absolutely refracted by it. He determined this both by a lens, and still more remarkably, by a prism of that substance.

With the prism interposed in the path of the rays coming from the source

* From the Greek, dia, through; and Oquare, to feel heat.

of heat, the effect was no longer transmitted in a straight line, but made to undergo a considerable deviation by the action of the prism. This was ob served to take place in different degrees, according to the nature of the source of heat. The greatest deviation took place when the flame of a lamp was employed; the next was incandescent platinum; the next with copper, at 390° centig., and when a vessel of boiling water was substituted, the effect was found too feeble to allow of any comparison with the other

cases.

However, when the rock-salt was cut in the form of a lens, the concentration of the rays, even from boiling water, was sufficient to give a decided proof of their being really brought to a focus.

Questions relating to the transmission of radiant heat through different media, are those which have formed the principal subject of M. Melloni's inquiries. But he has also, in one instance, directed his attention to the equally curious and important question, of the relation of the state of the surfaces and colour of bodies to heat. The instance referred to is an examination of the combination of the effect of a screen with that of surfaces, the very same, in fact, which constitutes the experiment first proposed and tried by Mr. Powell, and published in the Phil. Trans. for 1825. This fundamental experiment M. Melloni has repeated, and has completely verified it with his extremely accurate apparatus; a confirmation the more valu able, as some previous experimenters, since the date of the publication of the original investigation, seem to have overlooked it. It decisively proves that that portion of the heat from a flame, which passes through a glass screen, is also distinguished from the part which is intercepted, by the additional characteristic of affecting a black and a white surface in a different ratio. This is an inquiry eminently deserving to be followed up by the same method, and to be extended to a long series of different sorts of coating applied to the thermometer or thermo-multiplier.

Polarization of Heat. PROFESSOR FORBES.-M. Melloni failed in obtaining any detection of the effects due to the polarization of heat, which had been originally stated by M. Berard, though subsequent inquirers had been unable to discover any traces of it.

That zealous and highly-talented experimentalist, Professor Forbes of Edinburgh, here took up the subject. We believe we may say he was the first to introduce the use of Melloni's instrument into Great Britain. He has certainly employed it with signal success. Of such delicate inquiries as those respecting polarization, involving, in fact, complicated arrangements, which could hardly be made intelligible without lengthened details, it would not be possible to speak, in so rapid a sketch as the present, in a way to do them justice. But we must mention, however briefly and im perfectly, the valuable conclusion to which Professor Forbes's labours have led. In an elaborate and masterly paper in the Edin. Trans. (vol. xiii.), he has detailed these important researches. The analogies afforded by the polarization of light, led him to expect the most probable method of succeeding in the use of piles of mica; and the result fully justified his expec tations.

Two piles of plates of mica were placed obliquely in the path of the rays, so that the inclination was that of the angle requisite for polarization. In such an arrangement, it is well known that in one position of the second pile all light is stopped. The same was found to be true of heat; not only from flame, but even from non-luminous sources. This was not all; as in light, the interposition of a plate of crystal between the two parts of the appara

tus just described, restores or is said to depolarize, the light, so it was found to do with heat. On the principles of the undulatory theory this is explicable, and even subject to calculation in regard to light: by showing that a similar calculation will apply, Professor Forbes has rendered it in the highest degree probable that the same theory will hold good for heat; and has even pointed out the principle for calculating the lengths of the undulations necessary to be supposed, which he shows will be greater than those for light. This exactly accords with Melloni's result of their being less refrangible. Lond. Mag. Pop. Sc., April.

Keith Medal to Professor Forbes. The Council of the Royal Society of Edinburgh, have awarded this medal to Prof. Forbes, for his experiments demonstrating the polarization of light. The Vice President, in his address on the delivery of the medal, announces that Prof. Forbes has succeeded in producing the circular polarization of heat, the first public announcement of this new result. Ibid, Adress of Vice Pres. Doct. Hope, &c.

STONE.

On the Prismatic Decomposition of Electrical Light. By Prof. WHEATThe following is a brief notice of the principle results stated in this communication: 1. The spectrum of the electro-magnetic spark taken from mercury consists of seven definite rays only, separated by dark intervals from each other; these visible rays are two orange lines close together, a bright green line, two bluish green lines near each other, a very bright purple line, and lastly, a violet line. The observations were made with a telescope furnished with a measuring apparatus; and to ensure the appearance of the spark invariably in the same place, an appropriate modification of the electro-magnet was employed. 2. The spark taken in the same manner from zinc, cadmium, tin, bismuth, and lead, in the melted state, gives similar results; but the number, position, and colours of the lines vary in each case; the appearances are so different, that, by this mode of examination, the metals may be readily distinguished from each other. A table accompanied the paper, showing the position and colour of the lines in the various metals used. The spectra of zinc and cadmium are charac terized by the presence of a red line in each, which occurs in neither of the other metals. 3. When the spark of a voltaic pile is taken from the same metals still in the melted state, precisely the same appearances are presented. 4. The voltaic spark from mercury was taken successively, in the ordinary vacuum of the air-pump, in the Torricellian vacuum, in carbonic acid gas, &c., and the same results were obtained as when the experiment was performed in the air or in oxygen gas. The light, therefore, does not arise from the combustion of the metal. Professor Wheatstone also examined, by the prism, the light which accompanies the ordinary combustion of the metals in oxygen gas and by other means, and found the appearances totally dissimilar to the above. 5. Fraunhofer having found that the ordinary electric spark examined by a prism presented a spectrum crossed by numerous bright lines, Professor Wheatstone examined the phenomena in different metals, and found that these bright lines differ in number and position in every different metal employed. When the spark is taken between balls of dissimilar metals, the lines appertaining to both are simultaneously seen. 6. The peculiar phenomena observed in the voltaic spark taken between different metallic wires connected with a powerful battery were then described, and the paper concluded with a review of the various theories which have been advanced to explain the origin of electric VOL. XVIII.-No. 1.-JULY, 1836.

6

light. Professor Wheatstone infers from his researches, that electric light results from the volatilization and ignition (not combustion) of the ponderable matter of the conductor itself; a conclusion closely resembling that arrived at by Fusinieri from his experiments on the transportation of ponderable matter in electric discharges. Trans. Brit. Assoc. Lond. and Edin. Phil. Mag.

Kater's Portable Altitude and Azimuth Circle. An account of the performance of a small circle of this kind in the hands of the inventor, is given by Mr. Galbraith. The arch is of but three inches radius. By eight observations the latitude of Capt. Kater's house was determined within about 4.8 seconds of what was considered to be the true latitude, having been obtained by more powerful instruments. Mr. Galbraith speaks of a six inch circle of his own, with a telescope magnifying about twenty times and with three verniers to each of the circles. The level is divided to three seconds. Accounts of the satisfactory performance of his instrument are given.

Jameson's Journal

Supposed discovery of a new small Planet. This discovery M. Cacciatore, director of the Observatory at Palermo, supposes he has made. He has not as yet succeeded in establishing it definitively. Ibid.

Heat conveyed by Springs. Professor Bishoff gives a number of examples in which the temperature of the interior of the earth is brought to the surface by springs, and others in which, taking their rise in cold situations, the springs bring with them to a considerable distance the low temperature of their origin. Thirteen fresh water springs near the glaciers of the Tyrolese Alps, and in the limit of their snows, had a temperature from 36° to 4349 Fah. Some fresh water springs at the foot of the upper glacier near Grindelwald in Switzerland, had a temperature of between 37° and 38°. The springs on St. Gothard 3587 feet above the level of the sea, were found by Wahlenberg and Van Buch, to have a temperature of 37°

On the contrary, four springs at the foot of the Great Eiger in Switzerland, had only a temperature of 42°. Van Buch found a spring near Neufchatel of which the temperature was only 40°, while the neighbouring springs were from 50° to 50°. Ibid.

Relative level of Caspian Sea and of the Ocean. The supposed existence of a region of dry land 18,000 square leagues in area, surrounding the Caspian Sea, and below the mean level of the ocean, has naturally excited the most lively curiosity. The fact was regarded for twenty years as established by a series of barometrical measurements made in 1811 by Professors Engelhardt and Parrot. The difference of level which these travellers assigned to the Caspian and the Black Seas amounted to about 350 feet. But Professor Parrot, having revisited the tract in 1829 and 30, soon found reason to doubt the accuracy of his former conclusions. He learnt that some Russian engineers had ascertained by careful measurements that the Don, at the place called Katschalinsk, where it is only sixty wersts distant from the Wolga, is 130 Paris feet higher than the latter river, and as the Don flows with much greater rapidity to the Black Sea than the Wolga does to the Caspian, the difference of level between the two seas, if any, must be considerably less than 130 feet. Parrot accordingly made a series of levellings from the mouth of the Walga to Zarytzin, 400 wersts up its course, and from the mouth of the Don to the like distance; and these observations gave as a result that the mouth of the Don was between three and four feet lower than that of the Wolga! So that, according to this measurement, if there is any difference between the levels of the two seas, the Caspian is the higher! Address of Pres. Lyell to Geolog. Soc. and Lond. and Edin.

Philos. Mag.

Effect of Forests upon the Temperature, A motion has been lately made in the Chamber of Deputies for the general clearing of the woods in France. M. Arago showed that the clearing of extensive woods may be attended with effects of various kinds-the climate may be affected in many ways. He proceeded:"To form a mean temperature in a given climate, there may be a very unequal distribution in the monthly temperature: it is from hence that Buffon conceived the idea of distinguishing temperate climes, from excessive ones. The climate of North America is now severe-that of Europe was equally so before it was cleared of forests. At those early periods the winters were much colder, and the summers much warmer than at present. You will perhaps be surprised to hear that a few centuries ago the summer heat in the vicinity of Paris was much greater than it is in our own times. This is a fact, however, which is proved by various documents; among others, by a charter allowing the vine-growers of Amiens to compete with other districts of France, for the honor of supplying the most perfect wines to the table of Philip Augustus. I do not suppose that any vine-growers of Amiens, at the present day, would set up the pretension of being able to supply the best wine to any one.

"A very extensive modification has occurred in the climate of that region of France-it has been the necessary consequence of clearing the woods." Lond. Jour. of Arts, April.

Progress of Civil Engineering.

Report of E. F. Gay, Engineer on the Columbia and Philadelphia Railway, upon Motive Power.

To the Board of Canal Commissioners of Pennsylvania:

GENTLEMEN-Having made report to the superintendent relative to the construction of works done, or contemplated to be done, on the railway, I proceed to comply with the further instructions of your secretary, by reporting to you the situation of the line, and the motive power upon it.

The superstructure of the railway has continued in an excellent condition during the past season; indeed, with the exception of the north track, on the eastern twenty-two miles of the road; (which is constructed chiefly of wood,) the permanent character of the railway is a sufficient guarantee that few repairs of importance will be required on it for many years to come. The substructure of the road is generally of a solid and substantial character; the viaducts are the only works which are liable to injury from the heavy travel over them. Three of the piers to the Little Conestoga via. duct, and one at Mill Creek, have been badly fractured, as they were defective in their original construction; they have, however, been properly secured with buttresses, and may now be considered in a safe condition. is probable that one other pier at Mill Creek viaduct will require the support of buttresses in the course of another summer, which work the supervisor has included in his estimate for repairs. All the other viaducts are in a good condition. As the fears of the traveling community have been frequently expressed in relation to the danger of these viaducts taking fire from the sparks emitted from the chimneys of the locomotives, it may be proper for me to remark that little fear need be entertained of fire from such a cause; this may be inferred from the fact that the cool atmosphere in the viaduct condenses the steam as it escapes from the exhaust pipes, and so

It