64 THE ADVANCE OF SCIENCE 



as smoke and steam, upwards. And, as 

 we further notice that the earth, below 

 our feet, is made up of heavy matter, 

 while the air, above our heads, is ex- 

 tremely light matter, it is easy to regard 

 this fact as evidence that the lower region 

 is the place to which heavy things tend 

 — their proper place, in short — while the 

 upper region is the proper place of light 

 things ; and to generalise the facts ob- 

 served by saying that bodies, which are 

 free to move, tend towards their proper 

 places. All these seem to be natural mo- 

 tions, dependent on the inherent facul- 

 ties, or tendencies, of bodies themselves. 

 But there are other motions which are 

 artificial or violent, as when a stone is 

 thrown from the hand, or is knocked by 

 another stone in motion. In such cases 

 as these, for example, when a stone is 

 cast from the hand, the distance travelled 

 by the stone appears to depend partly on 

 its weight and partly upon the exertion 



