440 THE PRINCIPLES OF SCIENCE. 



The Ambiguous Expression. Uniformity of Nature. 



I have asserted that a serious misconception arises from 

 an ambiguous interpretation of the expression Uniformity 

 of Nature. (Every law of nature is the statement of a 

 certain uniformity observed to exist among phenomena^) 

 and since the laws of nature are supposed to be invariably 

 obeyed it seems to follow that the course of nature itself 

 is uniform,(so that we can safely judge of the future by 

 the present^ This inference is supported by some of the 

 most profound results of physical astronomy. Laplace 

 proved that the planetary system was stable, so that 

 no one of the perturbations which planet produces upon 

 planet shall become so great as to cause a disruption, and 

 a permanent alteration in the planetary orbits. A full 

 comprehension of the law of gravity shows that all such 

 disturbances are essentially periodic, so that after the lapse 

 of millions of years the planets will all return to the 

 same relative positions and a new cycle of disturbances will 

 commence. 



As other branches of inquiry progress, we seem to gain 

 assurance that no great alteration of the world's condition 

 is to be expected. A conflict with a comet has long been 

 a cause of fear to some persons, but now it is credibly 

 asserted that we have passed through a comet's tail with- 

 out the fact being known at the time, or manifested by 

 any more serious a phenomenon than a slight luminosity 

 of the heavens. More recently still the earth is said to 

 have actually touched the comet Biela, and the only result 

 was a beautiful and perfectly harmless display of radi- 

 ating meteors. A decrease in the heating power of the 

 sun seems to be the next most probable circumstance 

 from which we might fear an extinction of life on the 

 earth. But calculations founded on reasonable physical 



