180 THE NEW KNOWLEDGE. 



ball or flatten a bullet. A dynamite charge exploded upon 

 a stone will shatter the stone before the air has time to 

 move away. Matter also hesitates to stop. It tends to 

 overshoot the mark. A bullet will fly many a yard after 

 it has left the gun. In the gun it was urged by force, but 

 after it has left the gun it not only continues on its way 

 without force but against force, the force of friction, until 

 eventually it is overcome. It is sometimes asked: "Is per- 

 petual motion possible?" It is not only possible but neces- 

 sary if there is no interfering or opposing force. The inert- 

 ness, or inertia, of matter is defined in the First Law of Mo- 

 tion: " A body at rest remains at rest and a body in motion 

 continues to move with a constant speed in a straight line, 

 unless acted upon by some external unbalanced force." 

 We may illustrate this law by the case of a body which is 

 in motion under a pair of balanced forces. Thus, a steam- 

 ship under the application of force does not instantaneously 

 assume its full velocity; it gathers way. 



After it attains a constant speed it is under the applica- 

 tion of no resultant force whatever, the force developed by 

 the engines simply overcoming the contrary force of the re- 

 sistance of the water. It is thus obeying the first law of 

 motion. On the cessation of the force from. the engines, the 

 hitherto balanced force of friction begins to be effectual 

 and the ship stops, not at once but gradually. Now 

 this property of inertia which causes the steamship to act 

 in this way belongs, also, to electricity. The inertia of elec- 

 tricity is called self-induction. When a current of electricity 

 is suddenly started in a wire it does not rise to its full 

 strength instantaneously. Just like the steamship it re- 

 quires a certain time to gather way and to rise to its full 

 strength. Again, when the current is suddenly broken, it 

 does not stop instantaneously but tends to persist. This tend- 



