52 THE POPULAR SCIENCE MONTHLY. 



ciple will serve as an adequate test of progress. Any branch of 

 knowledge becomes a science only when the relation between 

 cause and effect is rigidly established, and the capricious and 

 accidental are as rigidly eliminated. Comte found his test of 

 science in the power of prediction. There is no science, unless 

 under certain given conditions we can say precisely what will 

 happen. But this, I take it, is only another way of saying the 

 same thing : we can predict only when we have perceived the 

 causal relations. 



The most common affairs of life have not yet been reduced in 

 practice to a science. Bread-making, for example, is still a black 

 art. You put flour and water and yeast and salt and lard to- 

 gether, and do certain things to it, and then trust to the gods to 

 make it into bread. Sometimes they do and sometimes they don't. 

 Sometimes you have good bread and more often you don't. Yet 

 I once met a man, an ex-college professor, who said that he always 

 had good bread. His recipe was simple : he made the conditions 

 invariable and the results were likewise invariable. We have all 

 heard of the lady who, when her servant was out, put wood and 

 paper and coal together, applied a match, and then went upstairs 

 and prayed that she might have a fire. 



Practically we do not disapprove of this condition of affairs. 

 For the most part, it amuses us. 



But the less domestic sciences afford better illustrations of the 

 realization of the principle. In the hands of Kepler, for instance, 

 astronomy failed to be a science. With wonderful skill he ap- 

 plied his knowledge of conic sections to the motions of the plan- 

 ets. Yet he could offer no better explanation of these motions 

 than the suggestion that each planet was the chariot of an in- 

 dwelling, guiding spirit. We could predict nothing of these 

 imaginary charioteers, for the laws which might be presumed to 

 govern them were quite beyond the limits of investigation. But 

 with the introduction of the conception of universal gravitation, 

 the study of astronomy took rank as a recognized science, and its 

 observed phenomena were reducible to an orderly sequence of cause 

 and effect. It is true that gravitation itself remains as profound 

 a mystery as the charioteers of Kepler, and in substituting the one 

 for the other we have not explained the universe. But we never 

 hoped to do that. The superiority of gravitation lies in this, that 

 it is the cause of uniform and measurable effects. Under Kepler's 

 conception of things, the perturbations of Uranus might be as- 

 cribed to a little caprice on the part of the charioteer. Under 

 Newton's conception such a disposition of the irregularities would 

 be impossible. They could result only from the attraction of a 

 definite amount of matter acting at a definite distance. When 

 Adams and Leverrier had completed their calculations, Dr. Galle 



