284 BROWN— THE AGE OF THE EARTH. 



to have been drawn off by the close approach of some second body 

 which later moved away. Each of these hypotheses has many objec- 

 tions. But it may be stated that from these points of view we can 

 learn nothing definite or even approximate about the age of the earth. 



Another method of approach is through observation of the present 

 condition of the bodies in the solar system. For evidence we have 

 eight major planets, but it is very doubtful whether from so small a 

 number we can deduce any results of value. In fact, it is now well 

 known that differences in mass may produce very different conse- 

 quences in the history of bodies. Thus arguments drawn from the 

 Moon, Mars, Venus, or the other planets have never inspired very 

 much confidence. 



Still another method is a consideration of the present condition 

 of the earth combined with the theory of contraction and subsequent 

 loss of heat. Here we are on somewhat firmer ground, since we have 

 many observations which give information concerning the interior 

 condition of the earth. Amongst these may be mentioned the values 

 of the mean density and the surface density, the phenomena of pre- 

 cession, nutation, etc., the measurements of earthquake and seismic 

 waves, and measurements of the rigidity of the earth by various 

 methods, and more particularly by that lately developed at Chicago 

 by Michelson and his colleagues. From these phenomena we know 

 with fair certainty that the earth behaves like a solid body which has 

 approximately the rigidity of steel. It is sometimes assumed that 

 this shows that the interior of the earth consists of matter which 

 under surface conditions of pressure would be solid. Unfortunately 

 the argument is doubtful, because we know nothing of the condition 

 of matter under the pressures which it experiences at depths of one 

 hundred miles or more below the surface of the earth. It is, there- 

 fore, impossible to argue with any security concerning the tempera- 

 ture conditions in the interior of the earth from these observational 

 data. Lately Jeffreys has shown that under almost any theory of 

 evolution the earth must at one time have been sufficiently hot so that 

 all its materials were in a liquid state, understanding by this latter 

 phrase, a state liquid under surface conditions of pressure. 



Thus the astronomical evidence which can be furnished as to the 



