132 PROGRESS OF SCIENCE IN THE CENTURY. 



of motion ; he groups these together if they are 

 seen to be really the same; he uses instruments 

 to enable his senses to detect hidden motions, and to 

 measure these with accuracy ; he tries to find a short 

 descriptive formula of antecedent and sequence which 

 will fit the facts. The so-called laws of motion 

 are " brief descriptions of observed similarities," 

 as Prof. J. J. Poynting expresses it.* As his for- 

 mulae increase in number and precision, he often 

 finds it possible to combine several of them in 

 a more general formulae, which may be so secure, 

 that is so accurate a description, that it affords a 

 basis for safe prediction. 



Aim of Physics. " To take an old but never worn- 

 out metaphor, the physicist is examining the garment 

 of Nature, learning of how many, or rather of how 

 few, different kinds of thread it is woven, finding how 

 each separate thread enters into the pattern, and 

 seeking from the pattern woven in the past to know 

 the pattern yet to come. How many different kinds of 

 thread does Nature use ? So far, we have recognised 

 some eight or nine, the number of different forms of 

 energy which we are still obliged to count as distinct. 

 But this distinction we cannot believe to be real. The 

 relations between the different forms of energy and 

 the fixed rate of exchange when one form gives place 

 to another, encourage us to suppose that if we could 

 only sharpen our senses or change our point of 

 view we could effect a still further reduction. We 

 stand in front of Nature's loom as we watch the weav- 

 ing of the garment; while we follow a particular 

 thread in the pattern it suddenly disappears, and a 

 thread of another colour takes its place. Is this 

 a new thread, or is it merely the old thread turned 



* Address, Section A, Brit. Ass. Report for 1899, p. 616. 



