THE USE OE' HADLEY'S QUADRANT.
For the Fore-Observation. Bring the in dex close to the bottom, so that the mid dle of the Vernier's scale, or nonius, stand against 0 degrees. Hold the plane of the instrument vertical, with the arch downwards ; look through the right-hand hole in the vane, and direct the sight through the transparent part of the hori zon-glass, to observe the horizon. If the horizon-line, seen both in the quick-sil vered part, and through the transparent part, should coincide, or make one straight line, then is the glass adjusted ; but if one of the horizon-lines should stand above the other, slacken the screw in the middle of the lever, backwards or forwards, as there may be occasion, un til the lines coincide: fasten the screw in the middle of the lever, and all is rea dy for use.
To take the Sun's Altitude. Fix the screens above the horizon-glass, using either or both of them, according to the strength of the sun's rays, by turning one or both the frames of those glasses close against the plane or face of the instru ment; then your face being turned to wards the sun, hold the quadrant by the braces, or by either radius, as is found most convenient, so as to be in a vertical position, with the arch downwards. Put the eye close to the right-hand-hole in the vane, look at the horizon through the transparent part of the horizon-glass, at the same time sliding the index with the left hand, until the image of the sun, seen in the quicksilvered part, falls in with the edge of the horizon, taking ei ther the upper or the under edge of the solar image. Swing yourbody gentlyfrom side to side ; and when the edge of the sun is observed not to cut, but to touch the horizon-line like a tangent, the ob servation is made. Then will the de grees on the arch, reckoning from the end next your body, give the altitude of that edge of the sun which was brought to the horizon. If the lower edge was observed, then sixteen minutes, added to the said degrees, gives the altitude of the sun's centre ; but if the upper edge was used, the sixteen minutes must be subtracted.
To take the Altitude of a Star. Look directly up at the star, through the vane, and transparent part of the glass ; the in dex being close to the button : then will the image of the star, by refraction, be seen in the silvered part, right against the star seen through the other part. Move the index forward, and, as the image de scends, let the quadrant descend also, to keep it in the silvered part, till it comes down in a line with the horizon, seen through the transparent part, and the ob servation is made.
To make an Artificial horizon. Often, when the atmosphere is clear above, the horizon is so laden with vapours, as to prevent an observation being taken. In
such case, an artificial horizon is to be made thus : fill into any vessel, having a diameter of about three inches, and about half an inch deep, from one to two pounds of quicksilver, on which lay a metal spe culum, or a piece of plain glass, whose diameter may be about one-third of an inch less than that of the surface of quicksilver : in this the image of the sun may be seen distinctly. Sling the vessel so that it may remain level, and take an observation with a stained glass, which will subdue the great brilliancy of the re» flection. The observation thus taken, will be as correct as if taken by means of the natural horizon.
As refraction causes each ray of light to assume a curved direction, all objects, when observed, especially by means of instruments, appear with an excess of altitude beyond their actual height, The refractions, to be deducted, fol low : The latitude of any place is its distance from the equator, either north or south, and never can exceed ninety degrees. It is found by taking the altitude of the sun, or star, above the horizon, with a quadrant, when on the meridian (i. e. due north, or south) of the place of observation. The meridian altitude, corrected for the dip of the horizon, and refraction, and sixteen minutes, the sun's semidiameter added thereto, gives the altitude of his centre, which, being subtracted from gives the zenith-distance, or the number of de grees the centre of the object is from the point over your head ; with which, and. knowing how far the object is to the north or south of the equator, which is is called its declination, the latitude is found by the meridian altitude of any ce lestial object, as follows : 1. If the object be south when ob served, call the zetith-distance, south ; and vice versa. Then, if the zenith-dis tance, and the declination, be of contrary names, (that is, if the sun, or star, comes to the meridian in the north, and has south declination, or per contra), the zenith-dis tance, added to the declination, gives the latitude of the place of observation ; the designation will be north, or south, ac cording as the declination may be.
2 When the zenith-distance, and the declination, are of the same name, that is, when the sun, or star comes to the meridian in the north, and has north de clination; or per contra ; then subtract the lesser from the greater ; and the re snainder is the latitude.
This general rule decides whether it be north or south. When the declination is greater than the zenith distance, the la titude is of the same name with the decli nation ;. but if less, the latitude is on the opposite side of the equator. For fur ther particulars, see LATITUDE.