APPENDIX. 



eafy method of correding in this inflrument the effeds of 

 heat upon the mercury it contains. 



It will eafily be conceived, from the conftru6tion of the 

 fcale of this hygrometer, that if its cylinder of ivory was 

 fuddenly changed into glafs, the inflrument would become 

 a true thermometer, in which the interval between the 

 points, anfwering to melting ice and boiling water, would 

 be divided into forty parts. If, therefore, a thermometer, 

 with a fcale limilarly divided into forty parts between the 

 fixed points, be placed near the hygrometer, it will ihew 

 immediately the corre^ion to be made on that inftrument 

 for its variation as a thermometer ; with fome reftriftions, 

 however ; of which M. De Luc has given an account in 

 the paper he fent to the Royal Society on the fubjed of this 

 hygrometer. 



That part of the frame of the inftrument on which the 

 fcale is marked, is moveable; fo that, before obferving 

 the points at which the mercury ftands, it may be puflied 

 upwards or downwards, according as the thermometer has 

 rifen or fallen with refpedt to the point of melting ice : and 

 thus the indications of the hygrometer can at once be 

 freed from the errors which would arife from the difference 

 in the volume of the quickfilver, on account of the 

 different degrees of heat. 



Defcription. 



127 



