president's address. ()19 



disposal, is a very minute fraction of the distance to tlie earth's 

 centre. In the consideration of the condition of matter at o-reat 

 depths beneath the surface it is frequently assumed that the 

 observed rate continues, or even increases, and that, consequently, 

 extremely high temperatures exist towards the centre. The 

 earth's radius is roughly about 21,000,000 feet, and if the above 

 rate of increase were maintained throughout we should have at 

 the centre a temperature of about 420,000° F., while some 

 writers have not hesitated to estimate the probable temperature 

 at as high as 1,000,000^^ F. Matter of any kind with which we 

 are acquainted would, at such temperatures, be much above its 

 critical point, i.e., the temperature at which condensation is 

 possible, and hence would be in a state of vapour. It has been 

 considered that the enormous pressure to which everything at such 

 depths is subjected would render this gaseous matter more rigid 

 than solid steel, the gaseous molecules being in a state which might 

 be expressed as one of " gaseous solidity."* A careful review of 

 the evidence, and particularly of the masterly mathematical 

 examination of the subject by Lord Kelvin,! leads me to 

 the conviction that reasoning based on the assumption of a 

 uniform increase in temperature downwards is quite fallacious, 

 and that a maximum is reached at a comparatively moderate 

 depth. The conclusion arrived at by Lord Kelvin is that the 

 observed increase in temperature downwards is not maintained, 

 but falls off at such a rate that at a depth of about 600,000 feet 

 the rate of augmentation has fallen to only one-tenth of a degree 

 F. per 51 feet, while at about 800,000 feet it has practically 

 reached zero with a temperature of about 7,000" F., which 

 continues to the centre. 



As the process of cooling proceeded the growing earth would 

 become denser and denser until solidification ensued, and the more 



* Nature, 13th April, 1905 (Vol. 71, p. 559) and Uth May, 1905 (Vol. 72, 

 p. 30). 



t Thomson and Tait, Natural Philosophy, Vol. i., Appendix D; Hep. 

 British Assoc. Adv. Science, 1876, Reports, p. 204. 



