[ 92 ] 



that the point B will coincide with b, (which is done by unfcrew- 

 ing the fcrew s i, (fig. 2.) and fcrewing up j- 2,) then the plummet 

 will bring back the ftem C B from the place b to the line of gra- 

 vity C B, and at the fame time bring the diameter b C of the mirror 

 to the horizontal pofirion UN: and if the inftrument be made 

 with attention to the circumftances already mentioned, the change 

 of its centre of gravity (if any; from the plumb-line, made by 



alteration 



becaufe then the elliptic path of the image will be more eccentric, and it will be 

 eafier perceived in what part of the mirror the vertex / of the ellipfe is formed, 

 which it has been (hewn is the molt elevated part of it. 



sdly. That the perfon adjufting the mirror muft make it revolve a fuTl femicircle, 

 in order that both vertices of the ellipfe may be feen ; for this, by doubling the 

 greateft angle of aberration, may make it perceptible, though fingly the motion 

 and divarication of the images could not be obferved : for which purpofe the inftru- 

 ment muft be changed on the needle, and replaced with its other fide towards the 

 arm K which fupports it : and if the cafe or veflel, in which it is fufpended, could be 

 turned partly round, it would facilitate the adjuftment when done by the image of a 

 very remote objedl. 



3d]y. That an obfervation may be made accurately by this level, even when it has 

 a great error of adjuftment ; by taking the mean of the greatefi and haft altitudes of 

 the obferved obje£t, which are difcoverable by the greateft and leaft divarication of 

 the images during a femi-revolution of the mirror , which will be feen in that diameter 

 of it, wherein is alfo the tranfverfe diameter of the ellipfe : and in it is to be taken one 

 of the two altitudes, and, after turning the mirror half round, the other : the mean of 

 thefe would reduce the reflefted image to the centre of the ellipfe, and is therefore 

 the true angular diftancc or altitude. This advantage of the level did not occur to- 

 myfelf, but was fuggefted by Mr. Profeffbr Brinkley, whole regard to the interefta 

 of fcience inclined him to give much attention to this inftrument. 



