Hydraulics. That science which treats of water in motion, and the means of raising, conducting, and using it for moving machinery, or other purposes.
Hydrostatics. Treats of the weight, pressure, and equilibrium of fluids, when in a state of rest.
Hydrometer. An instrument used to ascertain the specific gravity of different fluids, which it does, by the depth to which it sinks when floating on them.
Image. The picture of any object which we perceive either by reflected or refracted light. All objects which are visible, become so by forming images on the retina.
Impenetrability. That property of matter, by which it excludes all other matter from occupying the same space with itself at the same time. If two particles could exist in the same space, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point.
Incidence. The direction in which a body, or a ray of light, moves in its approach towards any substance, upon which it strikes.
Inclined Plane. One of the six mechanical powers. Any plane surface inclined to the horizon, may be so denominated.
Inertia. One of the inherent properties of matter. Want of power, or of any active principle within itself, by which it can change its own state, whether of motion, or of rest.
Inherent Properties. Those properties which are absolutely necessary to the existence of a body; called also essential properties. All others are denominated accidental. Colour is an accidental—extension, an essential property of matter.
Latitude. Distance from the equator, in a direct line towards either pole. This distance is measured in degrees and minutes. The degree of latitude cannot exceed ninety, or one quarter of a circle. Places to the south of the equator, are in south latitude, and those to the north, in north latitude.