[. 425 ] 
of the heat on the mercury. At the top of this fcale 
is feen an index, over-againft another fmall fcale, 
marked upon the unmoveable part of the frame. 
The degrees of this fmall fcale are eightieth parts of • 
the fundamental line, and confequently immediately 
anfwer to the degrees of the thermometer on the 
fame frame (48). When the index points to o of 
the fmall fcale, the thread which indicates upon the 
tube of the hygrometer the point to which the mer- 
cury funk in the melting ice, anfwers likewife to o in 
the fcale of the hygrometer. This is the cafeex- 
prefted in the figure wherein the thermometer is ' 
likewile reprefented at o of its fcale. By firjffc ob- 
ferving the heat therefore, and conducting the index 
to the point of the fmall fcale, which anlwers to the 
aCtual degree of the thermometer,, the hygrometer 
will only indicate upon its fcale the degrees^pf the 
humor. For this fcale going through the fame va- 
riations that the heat occafions in the height of the 
column of mercury, the indications of the hygro- 
meter become juft the fame as they would be, if the 
heat always continued that of the point at which ex- 
treme humidity was fixed, viz. o of the common, 
thermometer.. 
The fcale of the hygrometer is carried to the pro- 
per point, by means of a knob fixed on a fmalL 
piece of hard wood or metal, fcrewed to the bottom, 
of the board, and which affords a freepaftage to the 
tube of the hygrometer,. 
Account 
