MOTION. 
or of uniform motion in a right line, till by 
some external force it be made to change 
its state ; for, as body is passive in receiv- 
ing its motion, and the direction of its mo- 
tions, so it retains them, or perseveres in 
them without any change, till it be acted 
on by something external. From this law 
it appears, why we inquire not, in philoso- 
phy, concerning the cause of the continua- 
tion of motion or rest in bodies, which can 
be no other than their inertia; but if a mo- 
tion begin, or if a motion already produced 
is either accelerated or retarded, or if the 
direction of the motion is altered, an in- 
quiry into the power or cause that pro- 
duces this change is a proper subject of 
philosophy. 2. The second general law of 
motion is, that the change of motion is pro- 
portional to the force impressed, and is 
produced in the right line in whicli that 
force acts. When a fluid acts upon a body, 
as water or air upon the vanes of a mill, or 
wind upon the sails of a ship, the accelera- 
tion of the motion is not proportional to the 
whole force of those fluids, but to that part 
only which is impressed upon the vanes or 
sails, which depends upon the excess of the 
velocity of the fluid above the velocity 
which the vane or sail has already acquired ; 
for, if the velocity of the fluid be only equal 
to that of the vane or sail, it just keeps up 
with it, but has no eflfect either to advance 
or retard its motion. Regard must always 
be had to the direction in which the force 
is impressed, in order to determine the 
change of motion produced by it : thus, 
when the wind acts obliquely with respect 
to the direction of a ship, the change of her 
motion is first to be estimated in the direc- 
tion of the force impressed ; and thence, by 
a proper application of mechanical and geo- 
metrical principles, the change of the mo- 
tion of the ship in her own direction is to 
be deduced. 3. The third general law of 
motion is, that action and re-action is 
equal, with opposite directions, and are to 
be estimated always in the same right line. 
Body not only never changes its state of it- 
self, but resists, by its inertia, every action 
that produces a change in its motion : 
hence when two bodies meet, each endea- 
vours to persevere in its state, and resists 
any change : the one acquires no new mo- 
tion, but what the other loses in ilie same 
direction ; nor does this last lose any force 
but w'hat the other acquires; and hence, 
though by their collision, motion passes 
from the one to the other, yet the sum of 
their motions, estimated in a given direc- 
tion, is preserved the same, and is unalter- 
able by their mutual actions upon each 
other. 
All motion may be considered absolutely 
or relatively. Absolute or real motion, says 
Mr. Maclaurin, is when a body changes its 
place in absolute space ; and relative mo- 
tion, is when a body changes its place only 
with relation to other bodies. From the 
observation of nature, every one knows 
that there is motion ; that a body in motion 
perseveres in that state, till, by the action 
of some power, it is necessitated to change 
it ; that it is not in relative or apparent mo- 
tion in which it perseveres, in consequence 
of its inertia, but in real or absolute mo- 
tion. Thus the apparent diurnal motion of 
the sun and stars would cease, without the 
least power or force acting upon them, if 
the motion of the earth w'as stopped; and if 
the apparent motion of any star was de- 
stroyed by a contrary motion impressed 
ui)on it, the other celestial bodies would 
still appear to persevere in their course. See 
Inertia. 
To make this matter still plainer, Mr. 
Martin observes, that space is nothing but 
an absolute and infinite void, and that the 
place of a body is that part of the immense 
void which it takes up or possesses : and 
this place may be considered absolutely, or 
in itself, in which case it is called the abso- 
lute place of the body ; or else with regard 
to the place of some other body, and then 
it is called the relative or apparent place of 
the body. 
Now as a motion is only the change of 
place in bodies, it is evident that it will 
come under the same distinction of abso- 
lute, and relative or apparent. All motion 
is in itself absolute, or the change of abso- 
lute space ; but, when the motions of bo- 
dies are considered and compared with 
each other, then are they relative and appa- 
rent only : they are relative, as they ai e 
compared to each other ; and they are ap- 
parent only, insomuch that not their true 
or absolute motion, but the sum or differ- 
ence of the motions only is perceivable to 
us. 
In comparing the motions of bodies, we 
may consider them as moving both the 
same way, or towards contrary parts : in 
the first case, the difference of motion is 
only perceived by us ; in the latter, the 
sum of the motions. Thus, for ejftimple, 
suppose two ships, A and B, set sail Ifom 
the same port upon the same rhumb, and 
that A sails at the rate of five miles per 
