HYDRAULIC PURCHASE. A machine for drawing up vessels on a slip, in which the pumping of water is used to multiply the force applied.
HYDRAULICS. See [Hydrology].
HYDROGRAPHER. One who surveys coasts, &c., and constructs true maps and charts founded on astronomical observations. The hydrographer to the admiralty presides over the hydrographical office.
HYDROGRAPHICAL CHARTS or Maps. Usually called sea-charts, are projections of some part of the sea and its neighbouring coast for the use of navigation, and therefore the depth of water and nature of the bottom are minutely noted.
HYDROGRAPHICAL OFFICE. A department of the admiralty where the labours of the marine surveyors of the Royal Navy are collected and published.
HYDROGRAPHY. The science of marine surveying, requiring the principal points to be astronomically fixed.
HYDROLOGY. That part of physics which explains the properties of water, and is usually divided into hydrostatics and hydraulics. The former treats of weighing water and fluids in general, and of ascertaining their specific gravities; the latter shows the manner of conveying water from one place to another.
HYDROMETER. An instrument constructed to measure the specific gravities of fluids. That used at sea for testing the amount of salt in the water is a glass tube containing a scale, the bottom of the tube swelling out into two bulbs, of which the lower is laden with shot, which causes the instrument to float perpendicularly, and as it displaces its own weight of water, of course it sinks deeper as the water is lighter, which is recorded by the scale.
HYGRE. (See [Bore] and [Eagre].) An effect of counter-currents.
HYGROMETER. An instrument for ascertaining the quantity of moisture in the atmosphere.