122 Transactions. — Miscetlaneotts. 



Aet. X. — The Cause of Gravitation. By T. "VVakelin, B.A.N.Z.Univ. 

 [Bead before the Southland Institnte, 1st September, 1880.] 

 The title to this paper is merely descriptive. Some philosophers ob- 

 ject to the word cause. The reader can supply any other word, as antece- 

 dent, which he may think more correct.* 



It is generally advisable to survey the ground from which we start in 

 making any enquiry. Some of the most eminent astronomers and physi- 

 cists, the reader will perhaps rightly consider, should be referred to for this 

 information. 



" The illustration of supposing the sun connected with the earth by a 

 steel bar, will serve to give us some notion of the wonderful connection 

 which that mystery of mysteries, gravitation, establishes between them. 

 The sun draws or pulls the earth towards it. We know of no means of 

 communicating a pull to a distant object more immediate, more intimate, 

 than grappling it with bonds of steel. The velocity of sound, or of any 

 other impulse, conveyed along a steel bar, is about sixteen times greater 

 than in the air. Now suppose the sun and the earth connected by a steel 

 bar. A blow struck at one end of the bar, or a pull applied to it, would 

 not be delivered — would not begin to be felt — at the sun till after a lapse 

 of three hundred and thirteen days. Even light, the speed of which is 

 such that it would travel round the globe in less time than any bird takes 

 to make a single stroke of his wing, requires eight and one-third minutes to 

 reach us from the sun. But the pull on the earth which the sun makes is 

 instantaneous, or at all events incomparably more rapid in its transmission 

 across the interval than any solid connection would produce, and even 

 demonstrably far more rapid than the propagation of light itself."! 



" The opinion of most leading astronomers is that the velocity of 

 the gravitational pull, so to speak, is infinite — that is, it is instantaneous. 

 If it were not so, the members of the solar system would get beyond con- 

 trol, and the whole system would run into disorder. "J 



The law of gravitation is that every mass of matter attracts every other 

 mass with a force directly proportional to the mass, but inversely as 

 the square of the distance. The mass of a body can be determined by its 

 inertia. This would be the more rigorously exact and mathematical 

 method of determining the mass of a body. The determination of mass at 



* Even thus qualified this title is perhaps not strictly appropriate, 

 t Sir John Herschel : — " Lecture on the Sun." 

 I E. A. Proctor. — A note in one of his works. (The words are imperfectly given 

 from memory). 



