something relating to the night, in contradistinction to diurnal.

Nocturlabium, an instrument chiefly used at sea, in order to take the altitude or depression of some stars about the pole, and thereby to find the latitude and hour of the night. Some nocturnals are hemispheres or plumbispheres, on the plane of the equinoctial. Those commonly used amongst seamen are two; the one adapted to the polar star, and the first of the guards of the Little Bear; the other to the pole star, and the pointers of the Great Bear.

This instrument consists of two circular plates applied to each other. The greater, which has a handle to hold the instrument by, is about two inches and a half in diam- ter, and is divided into twelve parts, agreeing to the twelve months; and each month is subdivided into every fifth day, so that the middle of the handle corresponds to that day of the year in which the star observed has the same right ascension with the sun. If the instrument be fitted for two stars, the handle is made moveable. The upper left circle is divided into twenty-four equal parts for the twenty-four hours of the day, and each hour is subdivided into quarters. These twenty-four hours are noted by twenty-four teeth to be told in the night. Those at the hour of twelve are distinguished by their length. In the centre of the two circular plates is adjusted a long index, moveable upon the upper plate; and the three pieces, viz. the two circles and index, are joined by a rivet which is pierced in the centre with a hole, through which the star is to be observed.

To use the nocturnal, turn the upper plate till the long tooth marked twelve be against the day of the month upon the under plate; then, bringing the instrument near the eye, suspend it by the handle with the plane nearly parallel to the equinoctial, and viewing the pole star through the hole of the centre, turn the index about, till, by the edge coming from the centre, you see the bright star or guard of the Little Bear (if the instrument be fitted to that star); then the tooth of the upper circle, under the edge of the index, is at the hour of the night on the edge of the hour circle, which may be known without a light, by counting the teeth from the longest, which is for the hour twelve.