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APPENDIX. 



" The perpendicular rifing or falling of the mercury is 

 ** meafured by divifions, on a plate divided into inches and 

 " tenths, and by a Vernier divifion into hundredths of an 

 " inch, which is fixed to the fide of the tube." 



The Hygrometer I was favoured with by M. De Luc; 

 and the following account is a literal tranflation of that 

 which he gave me in French. 



The part of M. De Luc's Hygrometer which is affedled 

 by the impreffions of the moiihire of the air, is a hollow 

 cylinder of ivory, tv.'o inches eight lines long, and inter- 

 nally two lines and a half in diameter. It is open only 

 at one end ; and the thicknefs of its fides, for the length 

 of two inches fix Hnes from the bottom, is but three- 

 fixteenths of a line. It is this thin part which does the 

 office of an hygrometer; the remaining part of the 

 cylinder, towards its orifice, muft be kept a little thicker, 

 being deftined for joining it to a tube of glafs, thirteen or 

 fourteen inches long. This jundion is effected by means 

 of a piece of brafs, and the whole is cemented together 

 with gum lae. 



M. De Luc's reafon for chufing ivory as the hygro- 

 meter, is, that this matter appeared to him more proper 

 than any other for receiving the impreffions of the moifture 

 of the air, without fuffi^ring thereby any eflential change. 

 3 The 



