2 REST AND MOTION 



water has sunk to the bottom, then, as has already been said, the 

 principle of uniformity of nature leads us to suppose that every 

 stone which at any future time is placed in water will sink to the 

 bottom ; and we can then announce, as a law of nature, that any 

 stone, placed in water, will sink to the bottom. 



That part of science which deals with the laws of nature is 

 called natural science. Natural science is divided into two parts, 

 experimental and theoretical. Experimental science tries to dis- 

 cover laws of nature by observing the action of the forces of 

 nature time after time. Theoretical science takes as its material 

 the laws of nature discovered by experimental science, and aims at 

 reducing them, if possible, to simpler forms, and then discovering 

 how to predict from these laws what the action of the forces of 

 nature will be in cases which have not actually been subjected to 

 the test of experiment. For example, experimental science dis- 

 covers that a stone sinks, that a cork floats, and a number of sim- 

 ilar laws. From these theoretical physics arrives at the simple laws 

 of nature which govern all phenomena of sinking or floating, and, 

 going further, shows how these laws enable us to predict, before 

 the experiment has been actually tried, whether a given body will 

 sink or float. For instance, experimental science cannot discover 

 whether a 50,000-ton ship will float or sink, because no 50,000- 

 ton ship exists with which to experiment. The naval architect, 

 relying on the uniformity of nature, on the laws of nature deter- 

 mined by experimental science, and on the method of handling 

 these laws taught by theoretical science, may build a 50,000-ton 

 ship with every confidence that it will behave in the way pre- 

 dicted by theoretical science. 



3. The science of mechanics. The branch of science known 

 as mechanics deals with the motion of bodies in space, and with 

 the forces of nature which cause or tend to cause this motion. 

 The laws of nature which govern the action of these forces and the 

 motion of bodies have long been known, and were reduced to their 

 simplest form by Newton. Thus we may say that experimental 

 mechanics is a completed branch of science. 



