[ io8 ] 



it is plain that unlefs it fhould receive fome injury, it will 

 never after require any adjuftment ; but if it (hould, it may eafily 

 be redified in the fame manner. 



To make the level more fteady in the time of obfervation, a 

 tripod ftand like that of a telefcope might be fixed to the bottom 

 of the cafe MN to fupport it; the feet of which might be 

 contrived to turn upward and reft againft the fides of the cafe, 

 when the inftrument is not ufed. I could defcribe a mode of 

 doing this, but can add no more to this paper, the length of 

 which demands from me an earneft apology ; obferving only that 

 as this level, if made on either of the two conftrudions here men- 

 tioned, may be adjufted to the utmoft accuracy, fo it alfo refills the 

 agitation of the wind as well as the fluid level covered with glafs 

 planes, unlefs the quickfilver in the latter be Jikewife fheltered 

 by a glafs plate laid on its furface, which if the quickfilver 

 Ihould happen to' be fpilled would be ufelefs ; and fuppofing the 

 plate to be of fuch uniform thicknefs and denfity, and to float fo 

 equidiftant from the depreffed margin of the quickfilver on 

 every fide, no air bubbles likewife intervening nor accidental 

 contad with the fides of the veflel, that it would naturally lie 

 parallel to the furface of the fluid, yet if this fliould accidentally 

 be fo overfpread with duft or foulnefs, as to raife one edge of 

 the plate, and deprefs the other, even fo little as the thirty-three- 

 thoufandth part of an inch, it might give an altitude one minute 



of 



