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fixed objed feen as above-mentioned appears alfo fixed, the 

 mirror is as perfedly horizontal as die plumb-line is vertical ; 

 fuppofing the furface of the mirror to be a true plane, (and 

 one muft be unlucky indeed in the choice of a piece of looking- 

 glafs if it be not fufnciently fo) ; and as it can by the adjufting 

 fcrews be regulated to as much precifion as the eye, even affifted 

 by a telefcope, can afcertain the contad of two images, and 

 alfo proves its own adjuftment, (by turning it round on its axis), 

 which can be farther verified by comparing angles taken by 

 refledion from it with thofe taken from the furface of a fluid ; 

 alfo as it does not feem likely to change its form after being 

 adjufted and the fcrews made firm ; it feems in this refped un- 

 exceptionable, and it may be adjufted the way before-mentioned 

 by obferving the image of any fixed objed, as the fun or moon, 

 the top of a mountain or building, a candle placed at the dif- 

 Unce of ten or fifteen feet, a wafer faftened on the glafs-pane 

 tT a window, &c. if when the mirror is. made to revolve rovind 

 tue axis AG, without any pendulous vibration of the weight, 

 the image Jirjl afcends towards the part of the mirror moft re- 

 mote from the eye, and then defcends, that part of the mirror 

 in which the image is feen moft elevated or culminating alfo 

 afcends or is raifed above the level ; and it will be lowered by 

 firft unfcrewing a little the adjufting fcrew which is neareft to 

 the moft elevated place of the image on the fame fide, and then 



fcrewing 



