MECHANICAL ENERGIES OF THE SOLAR SYSTEM. 71 
Suns atmosphere or 3th 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. 
At the commencement of this communication, it was shown that the heat ra- 
diated from the Sun is either taken from a stock of primitive solar heat, or ge- 
nerated by chemical action among materials originally belonging to his mass, or 
due to meteors falling in from surrounding space. We saw that there are suffi- 
cient 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 them- 
selves; and that whatever may have been their original positions or motions re- 
latively to one another 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 ve- 
locity 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, how- 
ever, 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. 
Tn 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 ele- 
ments; or actual, as of relative motions among them. It is altogether de- 
pendent 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 ap- 
preciably 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 Zo- 
diacal Light.” The store of energy for future sunlight is at present partly dy- 
namical, 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 
VOL. XXI. PART I. U 
