QUADRANT. 



feved, that it may come close down to 

 them, and is there divided into smaller 

 portions : this scale is called the nonius, 

 and shows the smaller divisions in a more 

 correct and obvious manner than could 

 be done by the quadrantal arc, on which 

 each degree is subdivided into no more 

 than three equal parts, of 20' each. 

 Now the nonius, being 1 divided into 21 

 equal parts, shows at what portions of 

 the arc the index cuts the division of 20 

 minutes; therefore it shows every mi- 

 nute. 



THE USK OF HADLET'S Q.UADRAKT. 



For the Fore-Observation. Bring 1 the in- 

 dex dose to the bottom, so that the mid- 

 dle of the Vernier's scale, or nonius, 

 stand against 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 

 througii the transparent part of the hori- 

 zon-glass, to observe the horizon. If the 

 horizon-line, seen botli 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 pail 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 your body gently from 

 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 r 



