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.

HYPERBOLA. One of the conic sections formed by cutting a cone by a plane
which is so inclined to the axis, that when produced it cuts also the
opposite cone, or the cone which is the continuation of the former, on
the opposite side of the vertex.

HYPOTHECA. A mortgage. In the civil law, was where the thing pledged
remained with the debtor.

HYPOTHECATION. An authority to the master, amounting almost to a power
of the absolute disposal of the ship in a foreign country; he may
hypothecate not only the hull, but his freight and cargo, for necessary
and urgent repairs.

HYTHE. A pier or wharf to lade or unlade wares at [from the Anglo-Saxon
_hyd_, coast or haven].

I.

I. The third class of rating on Lloyd's books, for the comparative
excellence of merchant ships. (_See_ A.)

ICE-ANCHOR. A bar of round iron tapered to a point, and bent as a
pot-hook; a hole is cut in the ice, the point entered, and the hawser
bent to the shorter hook; by this vessels ride safely till any motion of
the ice capsizes it, and then it is hauled in. The ice is usually
entered by a lance, w