A. Ross Clark, A. Buchan, G. H. Darwin 177 



on solid matter has been utilized by Aitken in constructing a 

 little instrument which allows us to count the number of 

 particles of solid matter contained in the air. He found 

 that even the cleanest air will contain about 20 particles per 

 cubic centimetre, while in London or Paris the number 

 generally rises to well over 100,000. 



The work of Sir George Howard Darwin (1845-1912) 

 may serve to illustrate how a geophysical problem which in 

 its main features is easily understood, is found to involve 

 the whole history of the Universe as soon as we pass from 

 the general explanation to the more detailed study required 

 to give accurate numerical results. That the tides of the 

 ocean are due to the gravitational attraction of the sun and 

 moon was known already to Newton, and it can be shown 

 without difficulty that the explanation agrees in a general 

 way with observations. But, if we wish to formulate a 

 mathematical theory, we must begin by simplifying the 

 problem, and assume the earth to be a rigid solid sphere 

 covered entirely by a layer of water having the same depth 

 everywhere. The statement of this problem is simple enough, 

 but its solution becomes already complicated when the com- 

 bined attractions of the sun and moon are considered. Yet 

 we are not anywhere near the real tides on the real earth. 

 The ocean does not cover the whole globe, it is not of 

 uniform depth, and the solid core of the earth is not 

 absolutely rigid, but appreciably yields to the disturbing 

 forces. When we try to take account of these complications, 

 even in the roughest manner, we see that there must be a 

 frictional effect tending to retard the rotation of the earth; 

 this involves a re-acting force on the moon, and it can be 

 shown that this must slowly drive it further away. Hence 

 we conclude that there must have been a time when the 

 moon was nearer, and the earth rotated more rapidly, and, 

 looking still further back, this brings us to the time when 

 the moon may have formed part of the earth and ultimately 

 separated from it. Can we form an approximate estimate of 

 that time ? Such are the questions which occupied George 

 Darwin during a considerable part of his life. The whole 

 problem does not, of course, affect the earth only, but 

 concerns every celestial body. It opens out the whole 



If 



