II THE PROGRESS OF SCIENCE 81 



exhibit a constant movement, regular in amount 

 and direction, for which no external cause appears. 

 Others, as stones and smoke, seem also to move of 

 themselves when external impediments are taken 

 away. But these appear to tend to move in oppo- 

 site directions : the bodies we call heavy, such as 

 stones, downwards, and the bodies we call light, at 

 least such 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 extremely 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 



i the facts observed by saying that bodies, which are 

 free to move, tend towards their proper places. All 

 these seem to be natural motions, dependent on 

 the inherent faculties, 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 



I travelled by the stone appears to depend partly on 

 its weight, and partly upon the exertion of the 

 thrower. So that, the weight of the stone remain- 

 ing the same, it looks as if the motive power 

 communicated to it were measured by the distance 

 to which the stone travels as if, in other words, 

 VOL. I. G 



