[ 95 1 



It muft be evident from this account, that no error can happen 

 in any obfervation made with this inftrument, when it can be 

 ufed at all in the abfence of high winds, if its equilibration is 



permitted, 



direflion of the pillars a n, be in the plane of vifion or parallel to it, and the interval 

 and pofition of the image in the mirror, with refpeft to that in the horizon-riafs, 

 fliould be remarked : the weight being then taken oft, the mirror fliould be reverfed, 

 and its oppofite fide placed over the arm K; and when the weight is replaced, the 

 fame diameter of it fliould be brought into the plane of vifion as before: the image in 

 the mirror will now be feen in its higheft or loweft pofition in this diameter as the 

 place It appeared in before was the contrary ; and the image in the quadrant fliould 

 be raifed or lowered, by moving the index, 'till this image is judged to be in the 

 middle between thofe two places of the image in the mirror ; and this latter fliould 

 then be brought, by altering the two adjufting fcrews, which are for this purpofe 

 placed direcSly under the pillars, to coincide with the image in the horizon-Tlafs. 

 By a few trials, rcverfing the mirror, and altering the place of the images, 'till that 

 in the quadrant is got into the middle between the extreme pofitions of the other 

 image (which is the centre of the elliptic path it defcribcs) and this latter image then 

 made to coincide virith the former, the aforefaid diameter of the mirror will be made 

 level; and the diameter at right angles to it will be alfo rendered fo, by turninrr the 

 mirror about on each fide and altering the other two adjufting fcrews as the confe- 

 quent elevation or depreflion of the image direfts. If a telefcope only, furniflied 

 with crofs wires, be ufed for adjuftmenf, inftead of a quadrant, then the diredions 

 here given with, refpeft to the image in the horizon-glafs in the quadrant, are to be 

 applied, as if fpoken of the centre of the crofs wires of the telefcope. A little 

 praffice will produce fuch a degree of dexterity in this operation, that the adjuftment 

 rtiay be efleacd in a vefy fliort time ; I have done it more tliair once (to the exaft- 

 nefs of lefs than a minute of a degree) in Icfs than ten minutes. The mirror will 

 be more eafily turned round its axis, without producing pendulous vibrations of the 

 weight, and adjufted, if the weight be hung by a thread about a foot long, faftened 

 to the looped firing t, and made to fwing thus in water contained not in its own 

 cafe, but in another veflel, 'till the adjuftment is finiflied : then the weight and level 

 may be hung as before. 



