212 KINETIC THEORIES OF GRAVITATION. 



for example, a one-pound spherical iron shot thrown to a distance from 

 our terrestrial globe,) its mass may be entirely neglected as a vanishing 

 quantity, and we have the simpler expression ^ as indicating the amount 

 of action exercised by our earth upon such a ball. 



No hypothesis failing to embrace each of these six requirements de- 

 serves consideration ; and any hypothesis fully covering them all, might 

 be expected to account equally for the quite incomparable actions of 

 elasticity, magnetism, affinity, and cohesion, before being entitled lo 

 acceptance as a just or comprehensive theory of molecular force. 



As the projectors of Icinetic systems of gravitation have almost invaria- 

 bly quite ignored the fourth of the above conditions, it is worth while 

 here to dwell somewhat upon this point. Swift as the earth's orbital 

 motion is, (upward of 18 miles in one second,) the velocity of light is 

 about ten thousand times greater, being 185,000 miles per second. And 

 yet the composition of these two velocities gives .a displacement or 

 '.'aberration" of the heavenly bodies, as seen from our earth, of about 

 20" of angle for the observed direction of the visual ray. A luminous 

 impulse emanating from the sun requires about 8^ minutes to reach the 

 earth. Were the gravitative influence supposed to be so much swifter 

 than light as to require but a single minute to pass through this dis- 

 tance, there would still be a corresponding gravity "aberration" of 2.4" 

 of angle. The eftect of this slight obliquity of traction would be an 

 acceleration of the earth's orbital velocity which would become measur- 

 able in a single year. 



This is a subject which has been very fully and carefully investigated 

 by astronomers; and the illustrious Laplace, when he found an unex- 

 plained minute acceleration in the moon's orbit, threw out the sugges- 

 tion that if the velocity of transmission of gravitation did not exceed 

 eight million times that of light, it would satisfactorily explain the 

 lunar anomaly. It is scarcely necessary to say that when he subse- 

 quently discovered the secular diminution of eccentricity in the earth's 

 orbit, at present continuing, (though slowly reaching its minimum,*) he 

 recognized the true cause of the moon's irregullirit|^, which no longer 

 permitted even the unimaginable limit of possible velocity he had pro- 

 visionally assigned for gravitative action. 



Arago has remarked : " Now^ if we apply to the perturbation the maxi- 

 mum value which the observations allow, when they have been corrected 

 for the known acceleration due to the variation of the eccentricity of 

 the terrestrial orbit, we find the velocity of the attractive forcL^ to 

 amount to Jifti/ millions of times the velocity of light."t 



* The miuiminu coceutricity will be reached iu about one '"precession" pcnud, or 

 y5,000 years hence. 



tPopnlar Astronomy, book xxiii, chap. 27, vol. ii, p. 469 of the English edition. To 

 represent the real meaning of this velocity, it may be put into the equivalent form, 

 that if gravity occupied the one hundred -thousandth of a second in passing from the 

 sun to the earth, it would be detected. Or, the time required to reach us from the 

 nearest star (distant in light-travel about three years) would not exceed two seconds. 



