HAIL

, or Hailstones, an aqueous concretion, usually in form of white or pellucid spherules, descending out of the atmosphere.

Hailstones assume various shapes, being sometimes round, at other times pyramidal, crenated, angular, thin, and flat, and sometimes stellated, with six radii like the small crystals of snow.

It is very difficult to account for the phenomena of hail in a satisfactory manner; and there are various opinions upon this head. It is usually conceived that hail is formed of drops of rain, frozen in their passage through the middle region. Others, as the Cartesians, take it for the fragments of a frozen cloud, half melted, and thus precipitated and congealed again. Signior Beccaria supposes, that it is formed in the higher regions of the air, where the cold is intense, and where the electric matter is very copious. In these circumstances, a great number of particles of water are brought near together, where they are frozen, and in their descent they collect other particles; so that the density of the substance of the Hailstone grows less and less from the centre; this being formed first in the higher regions, and the surface being collected in the lower. Accordingly, in mountains, Hailstones as well as drops of rain, are very small; and both agree in this circumstance, that the more intense is the electricity that forms them, the larger they are.

It is frequently observed that Hail attends thunder and lightning; and hence Beccaria observes, that as motion promotes freezing, so the rapid motion of the electrisied clouds may promote that effect in the air.

Natural histories furnish us with a great variety of curious instances of extraordinary showers of Hail. See the Philos. Trans. number 203, 229; and Hist. de France, tom. 2, pa. 339.

HALF-Moon, in Fortification, is an outwork having only two faces, forming together a saliant angle, which is flanked by some part of the place, and of the other bastions. See Demilune and Ravelin.

Half-Tangents, are the tangents of the half arcs. See Scale and Semitangents.