126 APPENDIX. 



to afcertain upon them the points of melting ice and 

 boiling water, and to take exaftly the diftance between 

 thofe points by any fcale at pleafure. That done, the 

 bulb of this preparatory thermometer mufl: be broken, and 

 the quickfilver it contains exadlly weighed. It is by 

 knowing the weight of this, together with the diftance 

 between the fixed points of the thermometer, that the 

 fcale of the hygrometer is determined. For inftance, let 

 the weight of the quickfilver be one ounce, and the 

 diftance between the two abovementioned points, one 

 thoufand parts of a certain fcale : then fuppofe that the 

 quickiilver in the hygrometer, to which this tube is to 

 be applied, weighs only half an ounce ; this will give a 

 fundamental line, conftfting of five hundred parts of the 

 fame fcale. The fundamental line, thus found, is applied 

 to the fcale of the hygrometer, beginning at zero^ and 

 meafuring it off about four times over, that the whole 

 variation of the inftrument may be comprehended. Each 

 of thofe fpaces being afterwards divided into forty equal 

 parts, gives fuch degrees as M. De Luc has found moft 

 convenient. In general terms, the length of the funda- 

 mental line of the hygrometer, muft be to the interval be- 

 tween the two fixed points of the preparatory thermo- 

 meter, as the weight of the quickfilver in the hygrometer, 

 is to the weight of the quickfilver in that thermometer. 



This proportion between the fcale of the hygrometer 

 and that of the preparatory thermometer, furnifhes an 



eafy 



