PRESIDENTS ADDRESS. 23 



in order to account for the existing heat gradient near the surface 

 an internal temperature of about 7000° F. is required.* This is 

 much higher tlian the tetuperatures calculated by Messrs. Stiutt 

 and Fisher as being adequate if the central heat is derived from 

 radium contained in the crust. In the one case we have a hot 

 centre cooling outwards, in the other a warm crust also radiatin<y 

 heat outwards, but maintaining its thermal equilibrium by the 

 pi'oduction of heat from radium. The temperature diiference 

 between the two conditions, for the central mass, is a physical 

 necessity, and the estimation of its approximate amount is simply 

 a matter of calculation. 



For our present purpose it does not matter whether we adhere 

 to the nebular hypothesis, which, since its enunciation by Laplace 

 and subsequent elucidation by later mathematicians, and notably 

 by Lord Kelvin, has been practically universall}^ accepted, or 

 adopt the accretion theory brought forward by Professor G. H. 

 Darwin in his Presidential Address to the British Association 

 in 1905. According to the latter theory, the earth was built up 

 by the gathe> ing of pre-existing planetoids from its orbital region 

 in space, Either theory is competent to provide ample heat, 

 which is all that is required in our present discussion. In the 

 remainder of this address I will speak of the original heat of the 

 €arth merely as gravitational. The planetoids are commonly 

 held to have to a large extent originated from the gravitational 

 disruption of former celestial bodies through these approaching 

 within critical range of one another. This supposition has the 

 merit, against the collision theory, of better explaining the 

 structure of stony and other meteorites, which could not have 

 resisted the inevitable fusion, or even vaporisation, following 

 actual collision. 



Professor T. C. Chamberlin, of Chicago, has worked out a very 

 ingenious development of the accretion theory which is full of 

 extremely suggestive ideas, but seems to me to be less convincing 



* Kelvin, Popular Lectures and Addresses, Vol.ii. 1894, p. 318. See also 

 these Proceedings, 1905, p. 6 19. 



