THERMOMETER. 205 



AB is filled with the ftrongeft fpirit of wine or alcohol, and 

 from B to E is filled with mercury. 



It will be evident, from infpedlion, that if the fpirit of wine 

 is expanded by heat, the mercury in the fmaller tube will rife, 

 and, if the fpirit of wine is contradled by cold, the mercury will 

 fall : And although they are both fubjedled to the prelFure of 

 the atmofphere, yet, as liquids are incomprefTible by weight in 

 any perceptible degree, neither the fpirit of wine nor mercury 

 will be altered in bulk by the different weight of the atmo- 

 fphere. 



FD is a fcale of brafs or ivory, about 67 inches long, divided 

 in the ufual way. 



E is a fmall conical piece of ivory or glafs, of a proper weight, 

 made to float on the furface of the mercury in the fmadler tube : 

 to which float is joined a wire, reaching to H, having a knee 

 bent at a right angle, which raifes one index, and deprefles ano- 

 ther index, according as the mercury rifes or falls, which wire 

 fhall be termed xht float -wire, 



II is^a glafs tube, 7 inches and a half long, clofed at top and 

 open at bottom, fo wide as to Aide eafily over the fcale, and, by 

 means of a brafs rim cemented to it, is made to fit exa(5lly to 

 the circular bafe of the fcale, fo that, when this tube is put on, 

 it covers the whole fcale and indexes, and defends them froni 

 wind or rain. This cover need not be taken off^, except when 

 the inftrument is to be prepared for an obfervation. 



The operation of the float and indexes will be better under- 

 ftood from fig. 2. which reprefents them of the full fize. 



FG is the fcale fixed to a circular piece of wood or brafs, 

 through which the top of the fmall tube is made to pafs. 



From G to K is a piece of the fmallefl harpfichord wire, or 

 rather of the fmallefl: gold wire, fl;retched along the fcale, fixed 

 at the ends by two brafs pins. 



LL 



