1 66 A Short History of Astronomy [CH. VI. 



some other assignable cause. Thus a stone thrown along 

 a road comes to rest on account of the friction between 

 it and the ground, a ball thrown up into the air ascends 

 more and more slowly and then falls to the ground on account 

 of that attraction of the earth on it which we call its 

 weight. As it is impossible to entirely isolate a body from 

 all others, we cannot experimentally realise the state of 

 things in which a body goes on moving indefinitely in the 

 same direction and at the same rate ; it may, however, 

 be shewn that the more we remove a body from the 

 influence of others, the less alteration is there in its motion. 

 The law is therefore, like most scientific laws, an abstrac- 

 tion referring to a state of things to which we may 

 approximate in nature. Galilei introduces the idea in the 

 Dialogue by means of a ball on a smooth inclined plane. 

 If the ball is projected upwards, its motion is gradually 

 retarded ; if downwards, it is continually accelerated. This 

 is true if the plane is fairly smooth like a well-planed 

 plank and the inclination of the plane not very small. 

 If we imagine the experiment performed on an ideal plane, 

 which is supposed perfectly smooth, we should expect the 

 same results to follow, however small the inclination of 

 the plane. Consequently, if the plane were quite level, 

 so that there is no distinction between up and down, we 

 should expect the motion to be neither retarded nor 

 accelerated, but to continue without alteration. Other 

 more familiar examples are also given of the tendency 

 of a body, when once in motion, to continue in motion, 

 as in the case of a rider whose horse suddenly stops, or of 

 bodies in the cabin of a moving ship which have no tendency 

 to lose the motion imparted to them by the ship, so that, 

 t.g. t a body falls down to all appearances exactly as if the 

 rest of the cabin were at rest, and therefore, in reality, 

 while falling retains the forward motion which it shares 

 with the ship and its contents. Salviati states also that 

 contrary to general belief a stone dropped from the mast- 

 head of a ship in motion falls at the foot of the mast, not 

 behind it, but there is no reference to the experiment 

 having been actually performed. 



This mechanical principle being once established, it 

 becomes easy to deal with several common objections to 



