106 THE ANATOMY OF SCIENCE 



fact been adopted in the more recent develop- 

 ment of Einstein's gravitational theory. 



At first sight the idea seems preposterous. 

 How can the helices in a space-time map of the 

 solar system, by any stretch of the imagination, 

 or by any elasticity in our use of language, be 

 called straight lines .^^ But did you ever try to 

 draw a straight line upon a relief map? You 

 will say that that cannot be done. But you 

 might at least draw a line which seemed a little 

 straighter than any other, and if you object to 

 calling it a straight line you might be willing 

 to call it the straightest line. We only know by 

 experiment that measurements with a meter- 

 stick are fitted by Euclidean geometry, or that 

 measurements with metersticks and clocks are 

 fitted by our larger but still flat non-Euclidean 

 geometry. How do we know that in the neigh- 

 borhood of a massive body our measurements of 

 position and time will conform to any simple 

 geometry.? The new theory claims that they do 

 not, but it sets up a more difficult geometry 

 which departs more and more from the simple 

 flat geometry as we approach nearer and nearer 

 to a large mass. In this new geometry the path 

 of every body is one of these "streets called 

 straight"; or, in other words, it is a geometry 



