MECHANICAL ENERGIES OF THE SOLAR SYSTEM. 67 
the meteors in remote space had velocities relative to the Sun not incomparably 
smaller than the velocity calculated above as due to solar gravitation. 
But it appears to me that the hypothesis of no sensible resistance until the 
« Sun’s atmosphere” is reached, or the Sun’s surface struck, is not probable ;* be- 
cause if meteors were falling in to the Sun in straight lines, or in parabolic or 
hyperbolic paths, in anything like sufficient quantities for generating all the heat 
he emits, the earth in crossing their paths would be, if not intolerably pelted, at 
least struck much more copiously by meteors than we can believe it to be from 
what we observe ; and because the meteors we see appear to come generally in 
directions corresponding to motions which have been elliptic or circular, and 
rarely if ever in such directions as could correspond to previous parabolic, hyper- 
bolic, or rectilineal paths towards the Sun. If this opinion and the first men- 
tioned reason for it be correct, the meteors containing the stores of energy for 
future Sun light must be principally within the earth’s orbit : and we actually see 
them there as the “ Zodiacal Light,” an illuminated shower or rather tornado of 
stones (HerscuEL, § 897). The inner parts of this tornado are always getting caught 
in the Sun’s atmosphere, and drawn to his mass by gravitation. The bodies in 
all parts of it, in consequence of the same actions, must be approaching the Sun, 
although but very gradually ; yet, in consequence of their comparative minuteness, 
much more rapidly than the planets. The outer edge of the zodiacal light ap- 
pears to reach to near the earth at present (HERSCHEL, § 897); and in past times 
it may be that the earth has been in a dense enough part of it to be kept hot, 
just as the Sun is now, by drawing in meteors to its surface. 
According to this form of the gravitation theory, a meteor would approach the 
Sun by a very gradual spiral, moving with a velocity very little more than that 
corresponding to a circular path at the same distance, until it begins to be much 
more resisted, and to be consequently rapidly deflected towards the Sun; then 
the phenomenon of ignition commences; after a few seconds of time all the 
dynamical energy the body had at the commencement of the sudden change is 
converted into heat and radiated off; and the mass itself settles incorporated in 
the Sun. It appears, therefore, that the velocity which a meteor loses in en- 
tering the Sun is that of a satellite at his surface, which (being 5 of that due 
to gravitation from an infinite distance) is 276 miles per second. The mechanical 
value (being half that of a body falling to the Sun from a state of comparatively 
slow motion in space) is about 32,500,000,000 ft. lb. per pound of meteoric matter ; 
hence the fall of meteors must be just twice that which was determined above 
according to Mr Warterston’s form of the theory, and must consequently amount 
to 3800 Ibs. annually per square foot. If, as was before supposed, the density of 
the deposit is the same as that of water, the whole surface would be covered 
* For a demonstration that it is not possible, see Addition No. 1. 
VOL. XXI. PART I. T 
