192 HISTORY OF SCIENCE. 



Law i. Every body continues in its state of rest or of uniform mo- 

 tion in a straight line, except in so far as it may be compelled by 

 impressed forces to change that state. 



Law ii. Change of motion is proportional to the impressed force, 

 and takes place in the direction of the straight line in which the force 

 acts. 



Law in. To every action there is always an equal and contrary re- 

 action, or the mutual actions of any two bodies are always equal and 

 oppositely directed. 



The beginner in science who reads these laws for the first time may 

 fail to obtain a just idea of their meaning. The first law appears to 

 oppose common experience as regards bodies in motion, and no direct 

 experimental demonstration of its' truth can be appealed to, since we 

 are unable to witness that continued uniform motion in a straight line 

 of which the law makes mention. We learn by experience, however, 

 that all the motions which we can produce are interfered with by cer- 

 tain " impressed forces," which modify and destroy them. We may 

 slide a stone along the ground for a short distance, but it soon comes 

 to rest ; but along a sheet of ice it will go much farther, and we soon 

 recognize that the smoother the surface the more nearly is the law 

 realized. We conclude, therefore, that if the retarding force of friction 

 could be abolished altogether, as well ac another retarding force we 

 know to be at work, namely, the resistance of the air, the body would 

 continue to move with a uniform velocity. 



It has been held by some that the first of Newton's laws of motion 

 amounts to nothing more than a definition of body and of inertia. In 

 like manner the second law has been considered as nothing more than 

 a definition offeree, since of force there is no other criterion or mea- 

 sure than the change produced in a body with regard to its motion. 

 There can, however, be no question of the merit and originality of the 

 conceptions of Galileo and of Newton in dynamical science, and of 

 the great utility of some such statements of its fundamental principles 

 as Newton has given in his three laws. The vast genius of Aristotle, 

 with all his power of dealing with verbal subtleties, failed completely, 

 as did also the more practical mechanical science of Archimedes, to 

 discover a single true principle of dynamics, whereas the two centuries 

 which have passed since the publication of the ' Principia" have not 

 been able to improve upon Newton's expression of these laws, as 

 axioms or simplest truths from which the whole science of dynamics 

 may conveniently be deduced. 



The discovery of the first two laws is, however, due to Galileo, 

 and the third was known in some of its cases to Wallis, Wren, and 

 others, before it was announced in all its generality by Newton. This 

 third law is perfectly clear in its statement, but presents a difficulty 

 to persons unacquainted with the facts. The ordinary observer always 

 thinks of the magnet as attracting the needle : the effect on the latter 



