descend towards the interior, or as the supernal pressure increases; nor can we say, whether or not some inferior stratum may not have long since attained a state of absolute stillness or coldness. Indications among the superficial strata of the action of heat from beneath, do not necessarily indicate a state of higher temperature at the central parts; but rather that these parts have already lost a quantity of heat proportional to that from which their central heat is inferred, the superficial heat noticed being nothing else than that which they have given out; and we cannot conceive any cause to restore it to them again, but the cooling of strata still more deeply seated. For explaining all the phenomena of solar and terrestrial heat, however, it is not necessary to assume that their central parts are or ever were composed of homogeneous atoms, possessed of an adequate specific heat. A stratum of such atoms extending to a certain depth would be sufficient, and that such has really been the case will, I trust, appear in the sequel of this work. The intrinsic temperature of the surface of our earth must be comparatively very low; but, the solar influence included, it may be stated, that it is that of water under a pressure of 15lb. on the square inch; for in relation to water our world is organized. Now, though the idea be gratuitous that water should be found in the sun, this might take place though that ponderous sphere possessed a temperature immensely superior to that of the earth. The boiling point of water depends entirely upon the pressure upon its surface. The pressure of the air removed, the heat of the hand is sufficient to cause it to boil very rapidly; while, by exposing it to great pressure, it appears to have remained in the state of water when the vessel containing it was red hot. If the pressure of the atmosphere of our earth, then, retain water in the liquid state till it attain a heat of 212° Fahr., or 100 cent., the pressure of the sun's atmosphere, which surely will be a very extensive one, may retain water on his surface in a liquid state, though the bed of his ocean were as hot as any one can suppose. Besides this, the increased gravitation of the aqueous particles themselves, may confine it to liquidity at a temperature far higher than we might readily imagine.

These suggestions, however, are merely curious, and are introduced to shew, that, before we can assert, that, at such or such a temperature, substances will be in certain conditions, all things must be considered. To speculate concerning the physical condition of other planetary bodies, is one of the most refined researches of Natural Philosophy, and not to be attempted by every one who wishes to know whether there are men in the sun and moon.

The arguments which have now been advanced to establish the vulgar opinion that the Sun is superficially very hot, and may continue to be so for an indefinite time, goes to assert, that the Moon, as to her intrinsic temperature of surface, must be colder than the Earth. The quantity of her specific heat is less in the ratio of her mass. Her compressing or condensing force which developes the superficial heat, is also less in the same ratio; and her surface, from which heat will be radiated, is much greater than that of the earth in relation to their bulks. All these circumstances should tend to make her intrinsically colder on the surface, which her rather smaller density, indicative, perhaps, of a greater internal specific heat, would not be able to balance. All the phenomenal seem to countenance this idea. The coldness of moonlight is a common remark, which ought to follow if she is really cold; for, according to the common phenomena of radiant heat, the immediate effect of the coldness of her rays would be to abstract heat or produce cold in terrestrial bodies exposed to her beams. This view seems also to be countenanced by the fact, that her image, in the focus of Vilett's mirror (in which the sun's image fused flint in a moment,) though intensely luminous, was as cold as the ambient medium. But if she be much colder than the earth, one of the phenomena most immediately resulting will be, that the water on her surface must only exist as ice, and therefore she can have no seas. Now, on the whole of her hemisphere which is visible to us, and with which we must content ourselves, there is no extent of surface sufficiently smooth to admit of the supposition that it

is liquid, and the very glacial aspect which she presents in the telescope, are all favourable to this view.

But even supposing that the intrinsic temperature of the planetary bodies is represented in a rude way by their masses, and, consequently, that the superficial heat of the sun is very great, it does not follow that his heat is directly the cause of the heat of the sunbeam. To suppose this, would, indeed, be to assume nothing more extravagant than is usually done; and it might be supposed that ample provision has been made already for a temperature of the sun's body adequate to radiate a heat, which, at our distance from him, shall still possess all the intensity of the fire of the sunbeam. But, as has been already stated, it is difficult to conceive how a mechanical tremor, whether of the atomic or subtile matter of the radiant medium, decreasing continually as the distance increases, could be propagated to such extreme distances. That it must decrease in the course of any one ray, follows from the expansion of the intervals between the rays, which remain quiescent. Unless, then, there were an aliquot synchronism between the movements of contiguous atoms, their movements would interfere, and coldness would result, and thus along the course of a ray perpetual stoppages would occur to the continuous propagation of solar heat.

The undulatory theory of light has, indeed, familiarised our minds with notions as to a perfect mechanical elasticity; and if we could receive, that a wave of radiant matter may be mechanically propagated from a fixed star like a wave of water on the surface of a lake, it will demand but a small effort to believe that the mechanical tremor of the elastic matter of the radiant medium, at the surface of the sun, is propagated as far as the earth. But we find very little countenance for the belief of such perfect clasticity from any terrestrial phenomena, and by these it is right to be guided as often as possible. If we were to suppose, as is commonly done, that the heat of the sunbeam incident on the earth, were continued in an increasing ratio, as the inverse of the square of the

distance, the temperature, even at the distance of many comets' perihelion passages, would be greater than any thing we can conceive. Newton calculated, that the heat which dry earth, on the surface of the comet of 1680, while at its perihelion, must have conceived from the rays of the sun (on the supposition that heat increased according to the law that has been mentioned) was 2000° hotter than red hot iron! This assigns a temperature to the sun which is quite inconceivable to belong to matter under any form in which it is known to us. We are not necessitated, however, nor even at liberty, to entertain such ideas. The phenomena countenance the view, that the heat of the sunbeam at the earth, (except in as far as an atomic tremor, of a certain quantity is proper to the transmission of solar light,) is, in reality, excited only by the impact of these beams upon the solid matter of the earth. A ray of white light, illuminating a surface, is like a hammer in the centre of a spiral spring, performing nearly 458,000,000,000,000 strokes in a second, the central part coming in contact with. the surface struck as many times every second.

Such a continuous percussion must certainly excite the heat of the stratum of atoms on which it is made, and the heat then generated must be propagated upwards along the ray towards the sun, and downwards into the substance on which it is incident, and the effect resulting seems adequate to account for all the temperature generated by the sunbeam. It makes it to depend however, on the intensity of the lumeniferous action of the sun; and it is remarked, that in arctic regions the extreme force of the solar radiation is accompanied by a proportional brightness in the sun's rays; while, in tropical climates, though the lumeniferous excitement were even greater, the calorific action would be less effectual, in consequence of the greater heat already existing in the surface on which the sunbeam is incident. This view also makes the temperature of the sunbeam depend upon the resistance to its continuity by some opaque body, and certainly all the phenomena are favourable to such a view. Thus, a pencil of rays may be made to converge to a focus, by a mirror, or

lens, and out of this focus, though, in its immediate vicinity, not the least heat can be detected; but, in coming in upon the sun's image, which will be constituted there as soon as any body is placed to receive it, such a fire is generated as to fuse flint. This heat in the focus, as appears from the experiments of Count Rumford, is, as we should expect, greater than that in the unreflected sunbeam, in the proportion of the greater number of rays impinging on the same area.

It has been already assumed, that the density of the radiant mcdium increases as we ascend in the atmosphere. The solar radiation, therefore, ought to produce a greater heat from this cause, not less than from the more intense lumeniferous excitement; now the observations on the heat of the sunbeam on lofty mountains, prove that it developes more heat when incident on an opaque body, than in the plains below. It ought also to happen, that if a very small body were completely insulated, so as not to lose its heat by radiation or conduction, it ought to become extremely hot in the sunbeam, in a ratio faster than the decrease of its mass, the form remaining the same; and in accordance with this view, Professor Robison found, that a thermometer laid on cork and down, (which do not cool bodies much by becoming warm themselves,) and insulated in a glass case, arose, in a summer day, to 237°; and Saussure raised the temperature of a piece of burned cork considerably above that of boiling water. By conducting these experiments with a very minute apparatus, perhaps results of a very surprising nature might be obtained, and such as would throw much light upon vegetable physiology. We can form no idea as to the temperature which certain molecules on the leaves of plants may attain in the sunbeam. Thus the particles on which their colour depends are enclosed in cells, where there must be perfect stillness, in as far as the external agitation of the atmosphere is concerned, and they seem to be attached only by one point to the walls of the cell. These walls are laminæ far more attenuated and unfit for conducting heat than down; and the processes of assimilation which go on there, may be conducted at a temperature far higher