ADJUSTING HADLEY's qUADRANT. 155 



limited in its life. For when an angle exceeding go 

 degrees is to be mealured, or when an altitude of the fun, 

 &c, exceeding 45 degrees is to be taken, by means of a 

 refleiling horizontal lurface, and fuch cafes frequently 

 occur, then we muft either have recourfe to the back- 

 obfervation, or to fome other inftrument. 



In order to remedy, in fome meafure, this Inconve- 

 nience, the fextant, without any glafies for the back-ob~ 

 fervation, has been introduced; but even this inftrument 

 is Incapable of mcaluring an altitude, by means of a re- 

 fledting horizontal furface, as above, when exceeding 60 

 degrees ; and this will be the cafe with the meridian al- 

 titude of the fun. In mod parts of the United States, 

 during four months in the year. 



Various methods of adjufting the glaffes for the back- 

 obfervation, have indeed been propoled ; but thefe are 

 either very inaccurate and troublefome, or Inapplicable 

 to the common oftant, and require fome appendage to 

 the inftrument, with which but very few have yet been 

 made. 



The following method of making this adjuftment will 

 not, I flatter myfelf, be found liable to any of the above 

 objedlions. — It may be made on land, in a few minutes, 

 at anytime of the day when the fun flilnes ; requires no 

 additional apparatus, but v»/^hat any perfon may readily 

 make for himfelf ; and the adjuftment may be relied upon 

 as equally accurate with that for the fore-obfcrvation. 



Defcriplion of the Ncccffliry Apparatus. 



Take a piece of plane glafs (a piece of looking-glafs 

 will do very well) — take the polifh oft' one fide of It, and 

 cement It, with the rough fide down, on the flat fide of 

 the fegment of a wooden ball. The ball may be about 

 three or four inches in diameter, and the piece of gla-fs of 

 about the fame dimenfions. Or the glafs may be cement- 



ed 



