8 PHYSICS. 
at H, moving in the manner described ; if they all acted at once, the body 
would still arrive at H, but only by passing from B to H. A boat being 
rowed across a river affords a good illustration of this law. If the head 
of the boat be kept always pointing right across the stream, the passage 
will be made exactly in the same time as if it had been across a pond 
of the same breadth ; but it will be found that the boat has floated down 
the stream just as far as it would have done had it been simply floating 
on the stream for the same time. 
In describing the second law of motion, we have spoken only of the 
effect produced on a body by a force acting upon it; but there is also 
another effect to be noticed in such cases. If a stone rolled along the 
ground come in contact with another, the latter will of course be dashed 
onward, but a change will also take place in the motion of the former ; 
it will either be stopped entirely, or be dashed to one side, or will 
continue to move in the same direction as before, but with less force. 
The effect of the rolling stone on the stone at rest is called the action, and 
that of the stone at rest on the rolling one, the reaction. 
Third Law of Motion.—TZo every action there is always an equal and 
contrary reaction; or, the mutual actions of any two bodies are always equal, 
and oppositely directed in the same straight line. If a person in a boat 
push another boat lying alongside, both boats are moved almost equally 
from where they were floating, thus shewing that the pushing reacts on 
the one boat as much as it acts on the other. 
The force with which a body moves is called its momentum, and depends 
on the weight of the body, and the velocity with which it is moving. A 
stone when rolled or thrown has greater force than a ball of wood of the same 
size would have, because the one is heavier than the other; similarly, a 
large stone has greater force than a smaller one. Instead of weight, it is 
more proper to speak of the mass of a body; and in order to understand 
the meaning of mass, it is only necessary to remember the definition of 
‘density ;’ for to say that a stone is denser than a piece of wood is the 
same thing as to say that it has more mass than a piece of wood of the 
same size. Ifa single atom of matter were moving at the rate of one foot 
per second, and if we take this as the measure of momentum, then, 
in a mass of many atoms, the momentum would be measured by the 
number of atoms; and if this mass were to move at the rate of 100 feet 
per second, its velocity would be 100 times the number of atoms greater 
than that of the single atom. The momentum of a body is therefore 
measured by the mass multiplied by the velocity. 
