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greatest of these numbers, multiplied by 1390 to reduce to footpounds, expresses only the 6000th part, according to Mr Waterston's theory, and, according to the form of the Gravitation Theory now proposed, only the 3000th part, of the least amount of dynamical energy a meteor can have on entering the region of ignition in the Sun's atmosphere. Hence a mass of carbon entering the Sun's atmosphere, and there burning with oxygen, could only by combustion give out heat equal to the 3000th part of the heat it cannot but give out from its motion. Probably no kind of known matter (and no meteors reaching the earth have yet brought us decidedly new elements) entering the Sun's atmosphere from space, whatever may be its chemical nature, and whatever its dynamical antecedents, could emit by combustion as much as 10 of the heat inevitably generated from its motion. It is highly probable that many, if not all, meteors entering the Sun's atmosphere do burn, or enter into some chemical combination with substances which they meet. Probably meteoric iron comes to the Sun in enormous quantities, and burns in his atmosphere just as it does in coming to the earth. But (while probably nearly all the heat and light of the sparks which fly from a steel struck by a flint is due to combustion alone) only 18000 part of the heat and light of a mass of iron entering the Sun's atmosphere or th of the heat and light of such a meteor entering our own, can possibly be due to combustion. Hence the combustion of meteors may be quite disregarded as a source of solar heat.

000

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At the commencement of this communication, it was shown that the heat radiated from the Sun is either taken from a stock of primitive solar heat, or generated by chemical action among materials originally belonging to his mass, or due to meteors falling in from surrounding space. We saw that there are sufficient reasons for utterly rejecting the first hypothesis; we have now proved that the second is untenable; and we may consequently conclude that the third is true, or that meteors falling in from space give rise to the heat which is continually radiated off by the Sun. We have also seen that no appreciable portion of the heat thus produced is due to chemical action, either between the meteors and substances which they meet at the Sun, or among elements of the meteors themselves; and that whatever may have been their original positions or motions relatively to one another [See note on page 3.]

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or to the Sun, the greater part of them fall in gradually from a state of approximately circular motion, and strike the Sun with the velocity due to half the potential energy of gravitation lost in coming in from an infinite distance to his surface. The other half of this energy goes to generate heat very slowly and diffusely in the resisting medium. Many a meteor, however, we cannot doubt, comes in to the Sun at once in the course of a rectilineal or hyperbolic path, without having spent any appreciable energy in the resisting medium; and, consequently, enters the region of ignition at his surface with a velocity due to the descent from its previous state of motion or rest, and there converts both the dynamical effect of the potential energy of gravitation, and the energy of its previous motion, if it had any, into heat which is instantly radiated off to space. But the reasons stated above make it improbable that more than a very small fraction of the whole solar heat is obtained by meteors coming in thus directly from extra-planetary space.

In conclusion, then, the source of energy from which solar heat is derived is undoubtedly meteoric. It is not any intrinsic energy in the meteors themselves, either potential, as of mutual gravitation or chemical affinities among their elements; or actual, as of relative motions among them. It is altogether dependent on mutual relations between those bodies and the Sun. A portion of it, although very probably not an appreciable portion, is that of motions relative to the Sun, and of independent origin. The principal source, perhaps the sole appreciably efficient source, is in bodies circulating round the Sun at present inside the earth's orbit, and probably seen in the sunlight by us and called “the Zodiacal Light." The store of energy for future sunlight is at present partly dynamical, that of the motions of these bodies round the Sun; and partly potential, that of their gravitation towards the Sun. This latter is gradually being spent, half against the resisting medium, and half in causing a continuous increase of the former. Each meteor thus goes on moving faster and faster, and getting nearer and nearer the centre, until some time, very suddenly, it gets so much entangled in the solar atmosphere, as to begin to lose velocity. In a few seconds more, it is at rest on the Sun's surface, and the energy given up is vibrated in a minute or two across the district where it was gathered during so many ages, ultimately to penetrate as light the remotest regions of space.

TABLE I. Forces and Motions in the Solar System.

Masses in pounds.

Distances from the Sun's
centre, in miles.

Forces of attraction
towards the Sun, in
terrestrial pounds.

Velocities, in miles per second.

Explanation of Tables.

Mechanical Energies of the Solar System.
The following Tables exhibit the principal numerical data regarding the

In Table I., the mass of the Earth is estimated on the assumption that its mean

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TABLE II. Mechanical Energies of the Solar System.

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has kindly communicated to the author. proportions to that of the Earth, according to data which Professor Piazzi Smyth density is five times that of water, and the other masses are shown in their true

computed on the hypothesis, that the moment of inertia of each sphere is equal the In Table II., the mechanical values of the rotations of the Sun and Earth are

square of its radius multiplied by only one-third of its mass, instead of two-fifths of its mass as would be the case if its matter were of uniform density. These two estimates are only introduced for the sake of comparison with other mechanical values shown in the Table, not having been used in the reasoning.

The numbers in the last column of Table II., showing the times during which the Sun emits quantities of heat mechanically equivalent to the Earth's motion in its orbit, and to its motion of rotation, were first communicated to the Royal Society on the 9th January, 1852, in a paper "On the Sources Available to Man for the production of Mechanical Effect." [Vol. I. Art. LVIII.] These, and the other numbers in the same column, are the only part of the numerical data either shown in the Tables, or used directly or indirectly in the reasoning on which the present theory is founded, that can possibly require any considerable correction; depending as they do on M. Pouillet's estimate of Solar Heat in thermal units. The extreme difficulties in the way of arriving at this estimate, notwithstanding the remarkably able manner in which they have been met, necessarily leave much uncertainty as to the degree of accuracy of the result. But even if it were two or three times too great or too small (and there appears no possibility that it can be so far from the truth), the general reasoning by which the Theory of Solar Heat at present communicated is supported, would hold with scarcely altered force.

The mechanical equivalent of the thermic unit, by which the Solar radiation has been reduced to mechanical units is Mr Joule's result-1390 foot-pounds for the thermal unit centigrade-which he determined by direct experiment with so much accuracy, that any correction it may be found to require can scarcely amount to 20 or 3 of its own value.

1

300

ADDITIONS (May 9, 1854), No. I. Conclusion of Physical Astronomy against the Extra-planetary Meteoric Theory.

Meteors which when at great distances possessed, relatively to the centre of gravity of the solar system, velocities not incomparably smaller than the velocity due to gravitation to the Sun's surface, must strike the surfaces of the earth and of the other planets not incomparably less frequently than equal areas of the Sun's surface, and with not incomparably smaller velocities, and consequently must generate heat at the surfaces of the earth and other planets not incomparably less copiously than at equal areas of the Sun's surface. But the whole heat emitted from any part of the Sun's surface is incomparably greater than all that is generated by meteors on an equal area of the earth's surface, and therefore is incomparably greater than all that can be generated at his own surface by meteors coming in with velocities exceeding considerably the velocity due to his attraction from an infinite distance. Hence upon the extra-planetary Meteoric Theory of Solar Heat the quantity of matter required to fall in cannot

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