the ambient air itself, that frigorific impressions were communicated. When the sky was in its most serene state, frigorific impressions of 40 or 50 millesimal degrees were indicated from every part of the hemisphere. Those proceeding from the zenith, and those from the surrounding parts, were exactly equal. It was thus ascertained that the action of a given section, or angular portion of the sky, is the same at every obliquity. Dr Wells had found, that the appearance of the least cloud or thickness in the atmosphere nearly destroyed the effect of cold radiation, and produced an approach to equality of temperature in the thermometers placed in contact with different sorts of surfaces. Mr Leslie's delicate apparatus shewed with greater precision that the effect was not entirely destroyed, but continued in a greater or smaller degree according to certain definite circumstances. With the erect spheroid, he found in cloudy weather, that the frigorific impression diminished in proportion as the humid mass floating in the atmosphere seemed to descend. When the sky was canopied with high fleecy clouds, the effect on the instrument might amount to 20 degrees; but when the vapours sank so low as to hover on the hilly tracts, the impression did frequently not exceed five. The effect, therefore, evidently depends on the altitude of the lowest range of clouds, and seems to result from the difference of temperature which prevails there, compared with that of the surface of the earth, or other situations in which the apparatus is placed. The same conclusion was drawn from another set of observations. In a calm day, when a mass of dark clouds was spread at no great elevation above the surface of the ground, the spheroidal apparatus indicated only five millesimal degrees in a vertical position, and still

marked the same quantity when depressed to an angle of 30 degrees above the horizon. But had this impression of five degrees penetrated directly through the clouds from the higher regions of the atmosphere, the oblique passage presenting a diameter so much greater, would have scarcely allowed one half of a degree to escape through the mass. The fact proved that the clouds acted as a perfect screen, absorbing or extinguishing all the hot or cold pulses which it received from above, and then acted in its turn downward, communicating pulses of its own as an independent radiating body. Clouds consist merely of dispersed aqueous globules, and their influence is illustrated by that of water in the fluid state. Mr Leslie inclosed a pyroscope in an inverted spheroidal cup, and suspended it a few feet above the ground, while the sky appeared clear and blue. He then passed a silver tray under it, which received the impressions from the sky, and by reflection transmitted them to the inverted instrument. The cold thus reflected, amounted to 25 degrees; but on pouring a sheet of water over the silver tray, the effect was absolutely and immediately extinguished. For conducting such meteorological observations, Mr Leslie has constructed an instrument on a fixed scale, not only in its thermometrical degrees, but also in the extent of reflecting surface, as proportioned to the surface of the sentient ball. This beautiful instrument will be found a valuable accession, not only to meteorology, but to physical science in general. He has termed it the thrioscope, from the Greek term agios, which, in reference to the atmosphere, signifies at once" clear, dry, and cold." The sensibility of this instrument is very striking; the liquor instantly falls and rises in the stem with every passing cloud. Some

of its variations are not quite accounted for; as of two days of equal apparent clearness for example, it will indicate 50 on the one day, and 30 on the other. The action is greatest in general under a clear and translucid atmosphere. But particular winds blowing at different altitudes seem to modify the effect.

Mr Leslie then proceeds to investigate more closely the causes of these phenomena. It occurred to him, that since pulses (which others call radiations) are darted from such various surfaces, and since the softness of the external coat and its humidity seemed vastly to augment their power, it was possible that they might be likewise excited from a boundary of air itself; that the air probably thus acted in two capacities in these phenomena; that is, both as an intermedium for transmitting pulsations which it has received from a body differing from itself in temperature, and giving out radiations of its own, depending entirely on its particular temperature. The fact was ascertained by the following simple experiment: In a room where a steady fire was kept up, the athrioscope was set on the inside of the window, and directed to the upper part of the opposite wall; the instrument stood at zero, because the temperature by which it was surrounded, and that of the places at a distance to which it was directed, were nearly the same. The window was then thrown open, and the instrument was surrounded by a body of cold air, in consequence of which a motion in the fluid took place, indicating an impression of heat, evidently caused by the excess of temperature of the remote air of the room above that which was now contiguous to the æthrioscope. The same thing is shown by the different indications of an athrioscope, according as it is placed on the floor of a

heated room and directed to the ceiling, or placed near the ceiling and directed downward to the floor, the upper strata of air being the warmest. The instrument placed on the floor and directed upward, shews an impression of warmth, but when placed in the upper part and directed downward, it shews an impression of cold. If the actions excited in the air of a room are made thus apparent, much more is to be expected from the diversified condition of the different strata of so vast a body as the atmosphere. Taking it to the height of two miles, including scarcely onethird of the whole, the difference between the temperature of its extreme boundaries will amount to 20 degrees of the centesimal scale, or 36 of Fahrenheit. But the order is the reverse of what takes place in a close room, the air of the upper regions being invariably colder than that which is nearer to the surface of the earth.

As the higher strata of the atmosphere thus radiate cold downwards, so the lower strata must radiate heat upwards. To measure these would require the athrioscope to be inverted, and furnished with a pendent differential thermometer. The instru ment in this form carried to the top of a lofty mountain, and directed to the plain below, would indicate a considerable impression of heat, nearly proportional to the quantity of as cent. Perhaps on the summit of Chimborazo, it might amount to twenty millesimal degrees; and in the same situation the upright æthrioscope might be expected to mark an impression of cold from above, just so much diminished. If this last did not happen, it might be considered as giving countenance to the idea, that the giving out of caloric by radiation from bodies exposed to the heavens, consists in the simple escape of caloric into regions alto

gether beyond the boundaries of the atmosphere. No opportunity, however, has yet occurred on a large scale, for making these interesting observations. The ascent of a balloon would afford the readiest mode of verifying and extending the the ories suggested by the general aspect of the facts.

The inverted æthrioscope likewise discovers the quality and measure of the radiations (or pulses, as Mr Leslie denominates them) which are projected from the ground. These, as measured within short distances in the air, are very feeble, seldom in this climate exceeding three or four degrees. In the progress of a bright day, as the ground grows warmer than the air, it excites hot pulses: but, as the sun declines, the effect gradually diminishes; till this again returns, increasing with a contrary character, when the surface of the earth has become relatively colder. Another effect we may also expect to find, depending on the situation in which this instrument is placed in a clear night, when the ground becomes cooled by radiation, that the athrioscope will shew the most powerful impressions of cold when held a little way from the ground, and that, when it is placed on the surface, it will indicate the most powerful impressions, when placed on a portion of the surface which is the least radiating, and consequently the least cooled, because here the bulb, which is not sentient, will participate less than in another situation in any cooling effect communicated by the conducting quality of the surface, so that the difference between the two bulbs of the differential thermometer will be the greater, and these differences are the degrees which that instrument, in the form of the athrioscope, is fitted to indicate.

We cannot entertain a doubt, that

this instrument, employed by scienti fic persons in different parts of the globe, will contribute to throw much new light on the laws of temperature, as regulating the phenomena of the different regions of the atmosphere, and we may even hope that, as it becomes afterwards improved, it may open scenes altogether new in the interesting but intricate and difficult science of meteorology.

ASTRONOMICAL OBSERVATIONS MADE FOR DETERMINING THE FIGURE OF THE EARTH.

The improvements made in astronomical observation, in consequence of the high perfection to which astronomical instruments have arrived, has gradually led, and still leads to the solution of important scientific problems, which at no distant period appeared to be beyond the limits of human power. The coincidence of various favourable circumstances con. tributes to give daily accessions to the knowledge which the world possesses of the laws of nature, among which, none of the least is the cooperation of scientific men embodied in regular societies, by which extensive communications are maintained, and the task of investigating nature so subdivided as to admit of being prosecuted with undeviating closeness and deliberation in each of its parts, by numerous individuals. Armed with all these advantages, some enlightened men have been lately employed in imparting to different subjects in astronomy, a precision which they had not previously attained. It is our duty to notice the exertions which have been made in the last year, 1818, by M. Biot, of Paris, to measure an arc of the meridian, of which he himself has published a short but very interesting

account. The determination of the size and figure of the earth,-the measurement of gravity at its surface, the connexion of these phenomena with the interior construction of the globe, with the disposition of the strata, and the laws of their densities, are to be numbered among these long enduring questions which learned societies alone could propose to encounter and resolve. The first exact measurement of a degree of the terrestrial meridian was made in France by Picard in 1760. Newton availed himself of it in order to establish the law of universal gravity. Two years after this, Richer, who was employed by the Academy of Sciences, on a mission to Cayenne, for purposes of astronomical research, discovered that his clock, which at Paris beat the seconds gradually, went more slowly as he approached the equator, and that it again went quicker by the same gradation in returning towards the north, so as to resume exactly its original motion at the point of his departure. This was known to arise from the different intensity of the action of gravity in these different parts of the earth's surface; for they had just discovered that the quickness of the oscillation of a pendulum augments or diminishes with the force of gravity which causes its motion. The observation of Richer thus proved that the intensity was different in different latitudes, increasing in going from the equator to the pole. Newton, in his Principia, connected all these results with the law of attraction. He shewed that the variation observed in gravity, disclosed a flattening of the earth at the pole, a circumstance which is observable also in the form of Jupiter, Saturn, and the other planets which turn on an axis. He attributed this flattened form to the uniform attraction of the portions

of every planet, combined with the centrifugal force of its rotatory motion. He took them as in a fluid state, and shewed how to calculate the flattening of a planet of a homogeneous mass, according to the intensity of the gravity at its surface, and the quickness of its rotation. This theorem as applied to the earth gave a variation of gravity but little differing from that observed by Richer, though somewhat slighter, shewing that the strata of the earth_became denser as we penetrate from the surface to the centre, a doctrine since demonstrated by Clairault. More extensive measurements, however, were thought requisite. An accession of accuracy was expected to be obtained from the measurement of the complete arc which traverses France from Perpignan to Dunkirk, a measurement intended at the time to serve as a sort of axis to a general map of France, with the execution of which Colbert had intrusted the Academy. But in the imperfect state of the instruments and astronomical

methods of that period, the arc itself was too short to make the influence of the flattening distinctly perceptible; and the small variations in the lengths of the degrees being easily lost in the errors of the observations, the differences which were found were in such a direction as would have led to the inference of an elongation, instead of a flattening at the poles. The Academy perceived that the question could not be clearly decided, without measuring two arcs of the meridian, near the equator, and near the poles, from which greater differences might be brought out.

In 1735, Bouguer, Godino, and La Condamine, went to America, where they joined the Spanish commissioners. Some months after, Clairault, Maupertuis, and La Mounier, departed for the north.

The results of these expeditions put the flattening of the earth beyond a doubt, but did not fix its absolute amount. The degree of Peru, compared with that of France, gave a slighter flattening than if the earth were homogeneous: the operation of Lapland indicated a greater. In this uncertainty, the lengths of the pendulum, which they were careful to measure, agreed with the flattening deduced from the operation of the equator; but the exactness of these measurements, especially in the operation of Lapland, was not such as could enable them to solve the difficulty. The proceedings of the best observers could not be more accurate than they were; but the instruments then constructed laboured under imperfections.

After an interval of fifty years, astronomical instruments having become more perfect, and the methods of observation more precise, the Academy resumed these great operations with all the means which could insure their success. In order to give them greater importance, it was resolved to take the very size of the earth as thus determined for the fundamental element of a system of general, and uniform measures. The organization of the Academy was deranged, while its name was discontinued, during the stormy part of the French Revolution. But in the midst of the political confusion, Messrs de Lambre and Méchain, furnished with new instruments which Borda had invented for them, began and continued, often at the risk of their lives, the most extended and exact measurement of the earth which had ever been undertaken. Although they had many difficulties to encounter, hey concluded it as well as they could have done in the bosom of the mest profound peace. The measurement of the pendulum was also

attended to. Borda invented for this experiment a method surpassing in exactness every thing previously suggested, and never since exceeded.

It was afterwards thought that the arc of the meridian might be continued a good many degrees to the south across Catalonia, and even prolonged to the Balearic Isles, by means of a very large triangle the sides of which should join these isles to the coast of Valencia. Méchain having surveyed and measured the first triangles, died in a small town of Valencia, and Messrs Biot and Arago were charged with the completion of the work, along with the commissioners of the King of Spain. They happily succeeded, though Arago was subsequently exposed to danger and detained in captivity for some time before his return to France. The results confirmed those of the arc of France. They also measured at their most remote station, the length of the seconds pendulum, after the method of Borda, Biot, and Matthieu, and repeated the same operation on different points of the arc comprised between Perpignan and Dunkirk. These observations gave for the flattening of the earth a value almost equal to that which M. de Lambre had already obtained, by comparing the arc of France and Spain with the degrees of the equator, calculated with fresh pains; also with the degree of Lapland, which Mr Swanberg, an able Swedish astronomer, had corrected by new observations, and finally, with an arc of many degrees which Major Lambton had measured with great accuracy in India.

Verified by these combined coincidences, the arc of France and Spain acquired a farther claim to become the foundation of a standard of mea