APPENDIX. 



The cylinder made of it becomes more capacious, in pro- 

 portion as it grows moifter. This is the fundamental prin- 

 ciple of the inftruraent: M. De Luc has fmce found, 

 that upon letting this cylinder lie fome time in water of 

 an uniform temperature, it fwells to a certain point, after 

 which it dilates no further. This circumftance furnill:ied 

 him with a maximum of humidity ; and, confequently, with 

 one point of comparifon in the fcale of the hygrometer j 

 and this point he has fixed at the temperature of melting 

 ice. For meafuring the difierences in the capacity of 

 this ivory cylinder, and thereby difcovering its different 

 degrees of moifture, M. De Luc makes ufe of quickdlver, 

 with which he fills the cylinder, and a part of the com- 

 municating glafs tube. The more capacious this cylinder 

 is, or, which is the fame, the moifter it is, the lower does 

 the mercury ftand in the glafs tube; and 'uke verfd. Now 

 M. De Luc has found, that the lowefl point to which it 

 can fink, is that where it flands when the ivory cylinder is 

 foaked in melting ice : he therefore names this point zero^ 

 in the fcale of his hygrometer ; and confequently, the de- 

 grees of this fcale are degrees of drynefs, counted from 

 below upwards, as the quickiilver rifes in the glafs tube. 



To give thefe degrees a determinate length, and thus 

 render the hygrometers capable of being compared with 

 each other, M. De Luc employs in conftrudting them 

 fuch glafs tubes as have been previoufly prepared, by being 

 made into thermometers, and filled with mercury, fo as 



to 



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