Methud of udjmt'tng Hadlnfs Sextant. 125 



Having adjusted the hack observation glass as for di fore 

 observation^ draw a pencil line from its centre to the centre 

 of the hole in Z, and from the centre of the index glass let 

 fall a perpendicular LM : this line in my instrument is 

 6'33333 inches. 



Now (jH being previously made equal to LM, and the 

 index set to zero on the limb, turn the back glass to its 

 proper situation, and adjust it as nearly as you can. 



The line GH is now to be changed for the following ap- 

 paratus : — In a scale of wood (lig. .'>.) about live inches 

 broad, a foot long, and an inch thick, let there be a slit 

 made* two or three inches long, and not exceeding l-30th 

 of an inch in breadth : at G*3333 inches from this slit let 

 a scale of inches divided into tenths begin both ways ; let a 

 telescope. A, magnifying any number of times, from three 

 ■or four to thirty, be so contrived as to move nearer or 

 further from the slit by means of a screw, and also perpen- 

 dicular to the plane of the scale ; let the telescope also carry 

 an index corresponding to the centre of the cross hairs with- 

 in it; opposite to the slit let there be a socket to hold a small 

 bit of candle. The whole will be easily comprehended by 

 inspecting fig. 5. 



Fix both instrument and apparatus on a plank, or in a 

 box open at both ends, so that GM may, for a reason to 

 be given below, be 176-SS734 inches. 



If GK exceeds GH, the angle ACB is then more than 

 a right angle, and the angle KB li is double the angle SCB, 

 for SCA is drawn to represent a right angle. 



But if G FC equals G H, the angle ACB is a right angle ; 

 and Kand II coinciding, the angle KBH vanishes of course; 

 SCB vanishes also, and coincides v.ith the right angle 

 SCA. 



But if GK is less than GH, so as that K fall on P, then 

 the angle AGB will fall within, or be included in the right 

 angle ACS, and the index line GB will take an angular 

 situation similar to that of the line CW, and the lingle PBII 

 will be double of the then angle SCW. 



The reason why KBH is double the angle SCB is: Sup- 

 pose a ray comes from G upon A, is thence reflected to B, 

 the index glass B being set at more than a right angle to 



Tlie inference is. that by increasingthc distarce of the two rcflecrors and 

 the tyc, and coiiliniug ihe observati'jn to a square about one deforce, or 

 di)ublc the iun's diameter, in breadth, tlie accuracy is considcrab'y in- 

 creased . 



* The »lit falh bdiind the iVarr.c in the engraving. 



AC, 



