RESULTS AND LIMITS OF SCIENTIFIC METHOD. 441 



data show that no appreciable change can be going on, 

 and experimental data to indicate any change are wholly 

 wanting. Geological investigations show indeed that there 

 have been extensive variations of climate in past times ; 

 vast glaciers and icebergs have swept over the temperate 

 regions at one time, and tropical vegetation has flourished 

 near the poles at another time. But here again the vicis- 

 situdes of climate assume a periodic character, so that 

 the ultimate stability of the earth's condition does not 

 seem to be affected. 



All these statements may be reasonable, but they do 

 not in the least establish the Uniformity of Nature in the 

 sense that extensive alterations or sudden catastrophes 

 are impossible. In the first place Laplace's theory of the 

 stability of the planetary system is of an abstract character, 

 as paying regard to nothing but the mutual gravitation 

 of the planetary bodies and the sun. It overlooks several 

 physical causes of change and decay in the system which 

 were not so well known in his day as at present, and it also 

 presupposes the absence of any interruption of the course 

 of things by conflict with foreign astronomical bodies. 



It is now commonly acknowledged by astronomers that 

 there are at least two ways in which the vis viva of the 

 planets and satellites may suffer loss. The friction of the 

 tides upon the earth produces a small amount of heat 

 which is radiated into space, and this loss of energy must 

 result in a decrease of the rotational velocity, so that 

 ultimately the terrestrial day will become identical with 

 the year, just as the periods of revolution of the moon 

 upon its own axis and around the earth have already 

 become equal. Secondly, there can now be little doubt 

 that various manifestations of electricity upon the earth's 

 surface depend upon the relative motions of the planets 

 and the sun, which give rise to various periods of in- 

 creased intensity. Such electrical phenomena must result 





