4 PHYSICS. 
and the resistance of the atmosphere. The motions of a body will continue 
in proportion as these influences are counteracted. Thus, a top will spim 
on the smooth plate of an air pump, under an exhausted receiver, for hours 
after being set in motion. A body opposes a certain resistance to the force 
attempting to overcome its inertia, so that every motion is conditioned, on 
the one hand by the intensity of the influencing force, and on the other by 
the force of resistance of the body: its mass. The mass of a body is the 
amount of matter of which it is composed. 
A body let fall from a height will descend till it meets some obstacle. 
This is produced by gravitation, another general property of bodies. The 
falling of a body is, however, not the only result of gravitation. But more 
of this hereafter. The direction of gravitation coincides completely with 
the direction of a body suspended freely from a thread, as, for instance, a 
plumb-line; this direction, therefore, is called perpendicular, plumb, or 
vertical: the surface of standing water, as will be learned hereafter, is per- 
pendicular to this elevation. From this mutual relation has been deduced 
the proposition, that the direction of gravity is always perpendicular to the 
earth’s surface. As, however, the earth’s surface, or the water surface, is 
that of a spheroid, the perpendiculars to it must be in the direction of the 
radii produced ; whence it follows, that the direction of gravitation always 
tends towards the centre of the earth. Hence vertical lines are not parallel 
to each other, a fact which becomes inappreciable at short distances. Ata 
distance of 600 feet, for example, the angle at the centre of the earth, between 
two perpendiculars, amounts only to about 61 seconds. 
The force of gravity is exhibited by pressure when opposed to a resistance. 
The magnitude of this pressure is termed weight, this increasing with the 
number of material particles of which the body is composed, so that as the 
mass of a body is always proportional to its weight, the latter serves as an 
expression of the former. 
There remains to mention, in conclusion, among the general properties of 
bodies, their density ; in other words, the proportion of their weight to their 
volume. All bodies have a certain density, which depends upon the mode 
of aggregation, and the material of their single atoms. This density is 
termed specific gravity. As it is necessary to have a standard to which all 
densities may be referred, the weight of pure water, in its greatest density, 
has been taken as the unit of reference. By the density, then, or specific 
gravity of a body, is to be understood the ratio which its weight bears to an 
equal volume of pure water. If a certain mass of iron weigh 7.8 lbs., while 
an equal volume of water weighs 1 lb., the specific gravity of the iron 
is said to be 7.8. More will be said hereafter as to the proper mode 
of determining specific gravities. 
178 
