NOCTURNAL, or NOCTURLABIUM, an in strument chiefly used at sea, to take the altitude or depression of some stars about the pole, in order to find the lati tude and hour of the night. Some noc tornals are hemispheres, or planispheres, on the plane of the equinoctial. Those commonly in use among 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 cf the great bear.
This instrument consists of two circu lar plates applied to each other. The greater, which has a handle to hold the instrument, is about two inches and a half diameter, and is divided into twelve parts, agreeing to the twelve months, and each month subdivided into every fifth day ; and so as that the middle of the handle corresponds to that day of the year wherein the star here regarded 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 subdivided into quarters. These twenty-four hours are noted by twenty-four teeth to be told in the night. Those at the hours twelve,
are distinguished by their length. In the centre of the two circular plates is adjust ed a long index, moveable upon the upper plate. And the three pieces, viz. the two circles and index, are joined by a ri vet, which is pierced through the centre with a hole, through which the star is to be observed.
" To use the Nocturnal," turn the up. per plate till the long tooth, marked twelve, be against the day of the month on 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 centt'e, turn the index about till, by the edge coming from the centre, you-see the bright star or guard of the little hear (if the instru ment be fitted to that star) : then that 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 count ing the teeth from the longest, which is for the hour twelve.