28 THE GROUNDWORK OF SCIENCE

motion in all directions, each particle pressing equally on all

those which surround it and being equally pressed on by

them.

In Hydrostatics, therefore, pressure in all directions, and

not only the pressure of gravity, is considered, with the

well-known consequence that the surface of tranquil liquids

is horizontal, and water will always find its own level, and

those concerning the sinking and rising and other motions

of solid bodies in liquids. Hydrodynamics, or Hydraulics,

deals with the motions of liquids (waves, running water,

etc., etc.), which are so complex compared with those

of solids, and the various machines the utilities of which

are due to the laws of moving liquids water rams, water

wheels, etc.

The science of aeriform fluids, i.e., Pneumatics, adopts the

hypothesis that such fluids are composed of particles which

repel each other, separating as far as they can but pressing

equally in all directions. Such fluids are, therefore, both

extremely elastic and compressible, but, like solids and

liquids, they have their due weight, inertia, momentum, etc.,

and, like liquids, they have their waves of motion. The

weight of the atmosphere is also treated of in its practical

applications through the barometer, siphon, pump, etc.

We may place next the sciences which treat of what are

called the physical energies of matter, both in their non-

manifest or potential condition (capable of doing work), and

in their active or kinetic state (actually doing work). The

first of these sciences is that which treats of Heat, its powers

of expanding bodies, its phenomena of conduction, convection,

radiation, absorption, reflexion, and refraction, and its rela-

tions to other physical energies. The science of Light deals

in turn with its wonderful velocity of motion, in waves of

various lengths, its aberration, reflexion, refraction, inter-

ference, polarization, etc., with the laws of Optics, and such

motion in all directions, each particle pressing equally on all

those which surround it and being equally pressed on by

them.

In Hydrostatics, therefore, pressure in all directions, and

not only the pressure of gravity, is considered, with the

well-known consequence that the surface of tranquil liquids

is horizontal, and water will always find its own level, and

those concerning the sinking and rising and other motions

of solid bodies in liquids. Hydrodynamics, or Hydraulics,

deals with the motions of liquids (waves, running water,

etc., etc.), which are so complex compared with those

of solids, and the various machines the utilities of which

are due to the laws of moving liquids water rams, water

wheels, etc.

The science of aeriform fluids, i.e., Pneumatics, adopts the

hypothesis that such fluids are composed of particles which

repel each other, separating as far as they can but pressing

equally in all directions. Such fluids are, therefore, both

extremely elastic and compressible, but, like solids and

liquids, they have their due weight, inertia, momentum, etc.,

and, like liquids, they have their waves of motion. The

weight of the atmosphere is also treated of in its practical

applications through the barometer, siphon, pump, etc.

We may place next the sciences which treat of what are

called the physical energies of matter, both in their non-

manifest or potential condition (capable of doing work), and

in their active or kinetic state (actually doing work). The

first of these sciences is that which treats of Heat, its powers

of expanding bodies, its phenomena of conduction, convection,

radiation, absorption, reflexion, and refraction, and its rela-

tions to other physical energies. The science of Light deals

in turn with its wonderful velocity of motion, in waves of

various lengths, its aberration, reflexion, refraction, inter-

ference, polarization, etc., with the laws of Optics, and such