4li Professor W. Thomson on the 



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 re- 

 sistance until the " sun's atmosphere '* is reached, or the sun's 

 surface struck, is not probable* j because 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 in- 

 tolerably 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 cor- 

 responding to motions which have been elliptic or circular, and 

 rarely if ever in such directions as could correspond to previous 

 parabolic, hyperbolic, or rectilineal paths towards the sun. If 

 this opinion and the first mentioned reason for it be correct, 

 the meteors containing the stores of energy for future sun^light 

 Inust be principally within the earth's orbit i and we actually see 

 them there as the "zodiacal light,'' an illuminated shower or 

 rather tornado of stones (Herschel, § 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 appears 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 phsenomenon of ignition commences ; after a few 

 seconds of time all the dynamical energy the body had at the 

 fcOmmencement 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 

 entering the sun is that of a Satellite at his surface, which (being 



-^ of that due to gravitation from an infinite distance) is 



276 miles per second. The mechanical value (being half that 

 bf a body falling to the sun from a state of comparatively slow 

 motion in space) is about 82,500,000,000 ft. lb. per pound of 

 meteoric matter ; hence the fall of meteors must be just twice 

 * For a demonstration that it is not possible^ see Addition No. 1. 



