1 62 GRAVITATION— THE EXPLANATION 



behave; a hydrodynamics of perfect fluids doing things 

 which it would be contrary to the nature of any material 

 fluid to do. But it seems to be only in geometry that 

 he has forgotten that there ever was a physical subject 

 of the same name, and even resents the application of 

 the name to anything but his network of abstract math- 

 ematics. I do not think it can be disputed that, both 

 etymologically and traditionally, geometry is the science 

 of measurement of the space around us; and however 

 much the mathematical superstructure may now over- 

 weigh the observational basis, it is properly speaking an 

 experimental science. This is fully recognised in the 

 "reformed" teaching of geometry in schools; boys are 

 taught to verify by measurement that certain of the 

 geometrical propositions are true or nearly true. No 

 one questions the advantage of an unfettered develop- 

 ment of geometry as a pure mathematical subject; but 

 only in so far as this subject is linked to the quantities 

 arising out of observation and measurement, will it find 

 mention in a discussion of the Nature of the Physical 

 World. 



