IJd I^hthdcL oj'tpdjustlng Hadleij's Sextant, 



AC, whatever excess in incidence A receives, the same 

 excess iii refler.tion also it transmits to GK; of course, 

 therefore, K B H is double S C B. 



The apparatus being thus explained, and the principles 

 established, let us now apply them practically to measure 

 the angle KB H. 



Let us first find the length of Bil or GM. 

 From the prop;;rty of the circle it is evident, that if the 

 radius be 57-29578 inclies, the sine of one degree will be 

 one inch, and the sine and tangent of angles less than ten 

 minutes have no discernible difference : therefore the sine 

 may be u?<:d fot the tangent: therefore 3'i3- 77468 inches 

 radius will give six inches for one degree. 



Now, six inches being divisible into sixty tenths, one- 

 terith of an inch will correspond to one minute : but, as 

 has been already observed, the angle KJ3H will measure 

 doiiblc the angle SBC 3 therefore half the radius with the 

 srame six inches sine, will measure out the minutes of the 

 antrle SBC at one-tenth of an inch for each minute. 



Then 176-88734 inches must be the length of BH or 

 GW. 



If this length be doubled, tiien each tenth will measure 

 thirty seconds. 



If tripled, the divisions will be twenty seconds each, 

 8cc. Sec. 



Having placed the instrument, and directed the telescope 

 along the "line GM to A, light the candle and look for its 

 reflection through the slit, and you will find three, five, or 

 seven lines of light, but the middle one the brightest ; bring 

 that into the centre of the telescope by screwing the tele- 

 scope nearer to or further from the slit, as occasion may re- 

 quire : then note at what division the index on the telescope 

 stands : suppose it at 4^ of the divisions to the right, the 

 instrument will measure all angles by back observation 

 4' \b" too much, vet subject to the laws of the back obser- 

 vation in the common way : thus an angle of 42° 52' 30" so 

 taken, must be iiiminished to 42° 48' lo", and this taken 

 from 180° leaves 137° 1 1' 45'', the true angle : and so in 

 other cases. 



If this mode be not approved of, another, perhaps as ac- 

 curate, may be tried. Set the index of the telescope to O 

 on the scale of the apparatus, and the index of the instru- 

 ment at O on the liii\b; then move the index of the instru- 

 ment along the limb till the middle bright light occupies 

 the centre of the telescope as before; and the angle of devia- 

 tion. 



