76 SCIENCE IN THE LAST HALF CENTURY. 



heads is extremely light matter, it is easy to regard this fact as evi- 

 dence that the lower region is the place to Avhich heavy things tend — 

 their proper place, in short — while the upper region is the proper place 

 of light things; and to generalize 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 ex- 

 ample, when a stone is cast from the liand the distance travelled by the 

 stone appears to depend partly on its weight and partly upon the exer- 

 tion of the thrower. So that the weight of the stone remaining 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) the 

 power needed to send it a hundred yards was twice as great as that 

 needed to send it fifty yards. These, apparently obvious, conclusions 

 from the every- day appearances of rest and motion fairly represent the 

 state of opinion upon the subject which prevailed among the ancient 

 Greeks and remained dominant until the age of Galileo. The publica- 

 tion of the " Principia " of Newton in 1686-'87 marks the epoch at which 

 the progress of mechanical physics had effected a complete revolution 

 of thought on these subjects. By this time it had been made clear that 

 the old generalizations were either incomplete or totally erroneous; that 

 a body, once set in motion, will continue to move in a straight line for 

 any conceivable time or distance, unless it is interfered with ; that any 

 change of motion is proportional to the "force" which causes it and 

 takes place in the direction in which that " force" is exerted, and that 

 when a bodj' in motion acts as a cause of motion on another the latter 

 gains as much as the former loses, and vice versa. It is to be noted, 

 however, that while, in contradistinction to the ancient idea of the in- 

 herent tendency to motion of bodies, the absence of any such spon- 

 taneous power of motion was accepted as a physical axiom by the mod- 

 erns, the old conception virtually maintained itself in a new shape. 

 For, in spite of Newton's well-known warning against the "absurdity" 

 of supposing that one body can act on another at a distance through a 

 vacuum, the ultimate particles of matter were generally assumed to be 

 the seats of perennial causes of motion termed " attractive and repulsive 

 forces," in virtue of which any two such particles, without any external 

 impression of motion or intermediate material agent, w^ere supposed to 

 tend to approach or remove from one another; and this view of the 

 duality of the causes of motion is very widely held at the present day. 

 Another important result of in vestigatiou, attained in the seventeenth 

 century, was the proof and quantitative estimation of physical inertia. 

 In the old philosophy, a curious conjunction of ethical and physical 

 prejudices had led to the notion that there was something ethically bad 

 and physically obstructive about matter. Aristotle attributes all irregu- 



