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SCIENTIFIC NOTICES.

PHYSICS.

1.-On the Thermal Effects of Fluids in motion. By Professor W. THOMSON and J. P. JOULE, ESQ.1

These researches were made on bodies moving through air with velocities carefully measured by a whirling apparatus. The thermometers in use were filled with ether or chloroform, and were so graduated as to exhibit changes of temperature in extremely small divisions of the centigrade degree. It was thus found that a thermometer having a bulb nearly one inch in length and a quarter of an inch in diameter, would have its temperature raised 1° centigrade by a velocity of 163-7 feet per second. Another thermometer with a much more voluminous bulb, had its temperature raised to a corresponding amount by a velocity of 183.5 feet per second. On wrapping the thermometers successively with paper and with metallic wires, the effect of motion on temperature was considerably increased. With wire the effect was quintupled at slow velocities, thus rendering manifest the influence of fluid friction.

The authors have, on several occasions, noticed the effect of sudden changes in the force of the wind on the temperature of a thermometer held in it. Sometimes the thermometer was observed to rise, at other times to fall, when a gust came suddenly on. When a rise occurred, it was seldom equivalent to the effect, as ascertained by the foregoing experiments, due to the increased velocity of the air. Hence they draw the conclusion that the actual temperature of a gust of wind is lower than that of the subsequent lull. This is probably owing to the air in the latter case having had its vis viva converted into heat by collision with material objects. In sheltered situations, such as one or two inches above a wall opposite to the wind, they observed that a thermometer indicates a higher temperature than it does when exposed to the blast.

1 Proceedings of the Royal Society, No. 27.

2.-On the Influence of Temperature on the Elasticity of Metals. By M. KUPFFER. And on the Thermal Effects of Longitudinal Compression of Solids. By J. P. JOULE, Esq.

The results obtained by M. Kupffer are printed in the Compte rendu of the Physical Observatory of St. Petersburg. He finds that heat influences both the transverse and torsional elasticity of wires and rods of different metals. The decrease of elasticity for every degree (Reaumur) of increase of temperature is calculated by a formula containing terms deduced by observing the oscillations of rods at different temperatures. Thus, for silver, he finds a decrease of elasticity of 0.000568; for wrought iron, 0.0004696; Platinum, 0.00020110; plate glass, 0.0001242; Swedish iron, 0.0004555; English rolled hoop iron, 0.0004416; copper, 0.0005570; lead, 0.0003035. With high temperatures the loss of elasticity became a little greater.

Mr. Joule finds that heat is evolved by compression, and absorbed on removing the compressing force, in every substance he experimented on. In the case of metals the results agree very closely with the formula in which the longitudinal expansion by heat under pressure is considered the same as the expansion without pressure. He found that the experimental results were generally a little in excess of those calculated, thus indicating what M. Kupffer's researches had already established, namely, that the elastic force of metals is impaired by heat. Professor Thomson

has appended some valuable remarks on the alterations of temperature accompanying changes of pressure in fluids, from which it appears that pressure generally increases in a slight degree the temperature of fluids, and that this increase is greater the higher the temperature of the fluid operated upon.

3. On the Electro-Dynamic Qualities of Metals. By Professor W. THOMSON.

The author had already communicated to the Royal Society3 a description of experiments by which he found that iron, when subjected to magnetic force, acquires an increase of resistance to the conduction of electricity along, and a diminution of resistance to the conduction of electricity across, the lines of magnetization. By some experiments made recently, he has ascertained that the electric conductivity of nickel is similarly influenced by magnetism, but to a greater degree, and with a curious difference from iron in the relative magnitude of the transverse and longitudinal effects. Thus, with the same magnetic force, the effect of longitudinal magnetization in increasing the resistance, is from three to four times as great in nickel as in iron, while the diminishing effect of the

2 Proceedings of the Royal Society, No. 27, p. 564.

3 Bakerian Lecture on the Electro-Dynamic Qualities of Metals, Feb. 27, 1856, in the Philosophical Transactions.

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In con

transverse magnetization is nearly the same in the two metals. nection with the comparison it may be observed, that nickel was found by Faraday to lose its magnetic inductive capacity much more rapidly with elevation of temperature, and must, consequently, as the author has elsewhere shown, experience a greater cooling effect with demagnetization, than iron at the temperature of the metals in the experiments above mentioned. Professor Thomson further observes, that it will be very important to test the new property for each metal at those higher temperatures at which it is very rapidly losing its magnetic property, and to test it at atmospheric temperatures for cobalt, which, as Faraday discovered, actually gains magnetic inductive capacity as its temperature is raised from ordinary atmospheric temperatures, and which, consequently, must experience a heating effect with demagnetization, and a cooling effect with magnetization.

The present experiments, from the oblong form of the specimens of the metals used, do not admit of founding a quantitative comparison upon them; but the author hopes before long to be able to make a strict comparison between the effects for iron at least, if not for nickel also, and to find for each metal something of the law of variation of the conductivity with magnetizing forces of different strengths.-Proceedings of Royal Society, vol. viii., No. 27, p. 550.

4.-Optics and Painting.

M. Jamin has published in the Revue des Deux-Mondes during the past year some remarkable and highly interesting views on the connection of optics with the art of painting. As Mr. Ruskin's views on landscape painting have been received with considerable favour in these countries, and as many artists are more or less tinctured with the opinions of the realistic school, we thought it might prove useful to give the following abstract of these views, which we translate from that published by the Abbé Moigno in Cosmos.

When an artist desires to imitate a scene containing unequally distributed masses of light and shade, he is obliged to attribute to each of them its real value. He must, therefore, measure, or at least estimate, the brilliancy of different objects or of different surfaces, and graduate them in his copy according to the same proportional scale as in the model. For this purpose, he possesses an eye more or less exercised, which, however, as in other men, is a powerless instrument for the exact comparison of luminous intensities. He is, moreover, obstructed by the imperfection of resources of the art of painting; for nature generally presents an absolute brilliancy that no colouring could imitate. Unable to make his picture as perfect as nature, he is forced to darken it; but, for accuracy, he should at least maintain harmony and proportion of lights; that is to say, weaken all the lights in the same proportion. On this con

'Nichol's Cyclopædia of Physical Science, article "Thermo-Magnetism".

dition alone will his representation be true and faithful. How far has this condition been fulfilled in the most celebrated pictures? in other words, how far are the master-pieces of art true to nature? This is the problem M. Jamin has proposed for solution by the aid of optical science. More fortunate than the painter, the optician, knowing the imperfections of the eye, has invented photometrical apparatus, by which he can compare the brilliancy of neighbouring objects, and numerically express their relative illumination. By the aid of such apparatus, for instance, he ascertains that the shadow of a stick cast upon white paper has a twentieth of the brilliancy of the portions directly illuminated by the sun. M. Jamin himself has invented one of these precious instruments, of which we shall try to give a general idea. Imagine a small telescope like a single-barrelled opera glass. By putting the eye at the front we see that its interior is divided by a partition. On looking at an object through one of the compartments, a neighbouring object can be seen through the other; and by turning the tube upon itself, the partition may be made to coincide with the line of separation of the two objects. Close to the eye the instrument carries a movable graduated circle. If, continuing to regard the two objects, you turn this circle, you will remark that one becomes more distinct, while the other darkens. Soon the darker object becomes extremely black, while the other attains its maximum brilliancy. The graduation of the circle is so arranged as to show the difference in brilliancy of the two objects by the number of divisions which this circle has to be turned from the zero (found as above) until the objects appear in the field of view with an equal degree of brilliancy. To understand this better, conceive the shadow of a house cast on a white wall. Let us direct the telescope on the boundary line of the shadow; we see in one compartment the brilliant surface, and in the other the shadow; let us now turn the circle until the two parts acquire the same brilliancy, or until we see an equally illuminated surface. The divisions in the circle will then show that the mark which stood at zero at the commencement of our experiment has moved to 20, showing that the illuminated part of the wall is twenty times more brilliant than the shaded portion. Had the wall been yellow, blue, or any other colour, we should have found the same result. Instead of the wall and the shadow of the house, we might consider the ground and the shadow of a tree, a sunbeam and a shadow cast anywhere, the lines of separation of light and shade in a landscape, of a building and the sky, of blue sky and a cloud, etc.-in every case we would have obtained numbers expressing the relative brilliancy of objects contiguous to the field of vision, provided always that the photometer be suitably modified, not only according to the brilliancy, but the tints or colourings of the contiguous objects.

Let us now suppose an artist to have reproduced in a landscape a wall with a shadow, a piece of ground with the shadow of a tree, etc., and let us try to investigate the truth of his representation. The operations are precisely similar to those already described when examining the relations of the objects themselves.

M. Jamin states that after having submitted to the test of his photo

meter a great number of pictures, he has arrived at the unforeseen result, that in almost all, the proportional relations of the lights differ from those of nature. Always, or almost always, the shadows are not sufficiently deep; light and shade in pictures have also different colouring, so that the photometer, such as described, cannot, as in nature, render the apparent brilliancies of objects exactly equal. A twofold incorrectness is thus everywhere indicated, incorrect proportions of lights, false imitations of tints. Had these deviations from nature been trifling, painting might be admitted to be an approximate imitation of nature; but they are on the contrary very considerable. In the simple case of a body illuminated by the sun, and a shadow cast upon it, the relations found in summer, winter, different hours of the day, fine and bad weather, have been extremely varied. In general the minimum value of the relation of light and shadow is 10, its maximum value 20. But when sunbeams in pictures are examined, we find the above relations comprised between 2 and 4, so that the brilliancy of the sun-light is incomparably weaker in the pictures than in the true landscapes. It is difficult to conceive how the eye can tolerate such considerable inaccuracies. Still, all landscape painters do not deserve this reproach in the same degree; the modern school has made great progress towards exactness; every one may remark that their pictures have deeper shadows and brighter lights; some pictures of Decamps, for instance, present luminous effects comprised within the limits assigned by nature.

The discordance between nature and art in night pictures is not less remarkable. If in one of these pictures, usually lighted by a murky lamp, we compare the light of the lamp with the best illuminated parts, we shall find a relation comprised between 20 and 30. By placing in a room a lighted candle and a sheet of white paper, the ratio of the light of the candle to that reflected from the paper, will be found to be 1500; the candle flame is thus 1500 times as luminous as the paper, while in a picture it is made scarcely 30 times as luminous.

In the most celebrated interiors of Granet, the sky is 4 or 6 times brighter than the window-sashes of the rooms. To test this relation, M. Jamin selected a room with newly-painted sashes, which presented some similarity with those represented in Granet's pictures. By placing his photometer before the window, he found the sky 400 times brighter than the sashes. M. Jamin admits from trial the impossibility of imitating nature in this matter.

Let us now consider a complete landscape: in the foreground, masses of earth, trees, or buildings; in the middle distance, similar objects, seen through a stratum of air, which forms a kind of luminous veil, and increases their brilliancy; in the background, mountains, which blend themselves with the sky; the clouds, whose light far surpasses that of terrestrial objects; the sun, finally, whose dazzling splendour no eye can bear. Measured by the photometer, the luminous intensity of the clouds is several thousand, sometimes several million, times as great as a tree close to the observer. What can the painter do to imitate the infinite gradations in such a scale, when his brightest white has only a