[ 43 ] 



of mercury with its convex furface againft the light, through 

 a groove cut intirely through the brafs tube ; within this latter, 

 and furrounding the glafs tube, Aides a fmall one, carrying 

 the vernier, and moved by tooth and pinion ; the lower edge 

 of this fliews an accurate contadt with the convex furface of 

 the mercury at the time of obferyation. 



Plate 2d, is a fedlon of the refervoir and other particuUrs. 



In fig. lit, RRRR is the refervoir formed in the turning lathe, 

 by means of an hollowing tool introduced through the hole 

 made for the piece L, which contains the glafs tube I. 



A is a cylindrical plug of wood, which is made to afcend or 

 defcend in a hollowed cylindrical fpace, which it very nearly 

 fills when down ; this afcent and defcent is cfFeded by turning 

 the piece M, on which is cut a fwift fcrew working in the 

 piece B, which piece B is fcrewed firmly into the box ; after 

 that the plug A is introduced and fitted into its place. G is an 

 ivory floating gage of the ufual form. 



Let us now fuppofe the mercury introduced into the refer- 

 voir, with the floating gage refting upon it, its furface being 

 reprcfented by the dotted line. If now the fcrew M be turned 

 fo as to raife the plug A, the mercury will defcend into the 

 deferted fpace, and the furface and the floating gage refting 

 upon it will defcend : Thus it appears that the piece A, as 

 raifed or deprcflTed by the fcrew M, is the regulator of the 

 gage at the time of making an obfervation. 



* Fa Now 



