Wakelin. — On the Cause of Gravitation. 123 



the surface of the earth is made, however, by simxjly weighing small bodies. 

 For comparatively small masses " it has been established by experiment, 

 that the two modes of comparing masses perfectly coincide."* The mass of 

 matter in a planet can of course only be inferred from the degree of gravi- 

 tational force exerted by that planet. 



The pull which the earth exerts upon, say a ton mass of iron at the 

 surface of the earth, is what we mean by the iveight of that mass. The 

 force of gravity at the surface of the sun — that is the gravitational pull — is 

 nearly twenty-eight times that at the surface of the earth. f The same mass of 

 iron at the surface of the sun would therefore weigh nearly twenty-eight tons. 

 If this ton mass of iron were placed at the distance of Mercury from the sun 

 the pull of the gravitational power of the sun would give this mass a weight 

 of only 9^ pounds. At the distance of the earth from the sun, the pull 

 exerted by the sun would give to this ton mass only a weight of 1^ pounds. 

 Under a vertical sun at mid-day, a ton mass at the sm-face of the earth, 

 owing to the pull exerted by the sun, would weigh in a spring balance 

 2|- pounds less than it did at midnight at the same place. It is this very 

 small difference in the force of gravity on opposite sides of the earth (where 

 mid-day and midnight), that causes the earth to gravitate to, and on 

 account of its motion to revolve around, the sun. 



The bearing on this enquiry of the next two or three extracts is not 

 perhaps very direct, but the extracts will aid us considerably in forming an 

 opinion of what the " gravific " force may be. 



" From this phenomenon (Faraday's lines of magnetic force) Thompson 

 afterwards proved by strict dynamical reasoning that the transmission of 

 magnetic force is associated with a rotatory motion of the small part of the 

 medium. He showed at the same time how the centrifugal force due to this 

 motion icould account for magnetic attraction. The explanation of electrostatic 

 stress is less satisfactory, but there can be no doubt that a path is now open 

 by which we may trace to the action of a medium all forces like electric and 

 magnetic forces." " Such a state of stress as is necessary to produce gravi- 

 tation we have not however been able hitherto to imagine." \ 



The a9ther fills space, and is necessary in the undulatory theory of light — 

 long accepted — to account for the transmission of light. "It interpenetrates 

 all the transparent bodies, and probably all opaque bodies too. We must 

 consider the aether in dense bodies as somewhat loosely connected with the 

 dense bodies, and we have next to enquire whether when these dense bodies 



* Deschanel's Natural Philosophy, " Mass," p. 55. 

 t Newcombe, 27'71 times. 

 \ J. C. Maxwell's article, " Attraction," in Encyclopffidia Britannica, ninth edition 

 (now being published). 



