[ 160 ] 
may be therefore ufeful, to reduce M. de luc’s 
rule to Englifh meafure, and to adapt it to the 
thermometer of Fahrenheit’s fcale, which is gene- 
rally ufed in this country. 
M. de luc, in the winter feafon, heated the 
air of his room to as great degree as he could, and 
noted the rife of the barometer, owing to the di- 
minution of its denfity, or fpecific gravity, by heat j 
he alfo noted the height of the thermometer, 
both before and after the room was heated. Hence 
he deduced a rule, that when the barometer is at 
27 French inches, which was the cafe in this experi- 
ment, an increafe of heat, from freezing to that 
of boiling water, will raife the barometer 6 lines, 
or -J-th part of the whole. It is eafy to fee, that 
when the barometer is higher than 2 7 French inches, 
this variation will increafe in the fame proportion ; 
or will be always th of the heighth of the ba- 
rometer; therefore, if the height of the barometer 
be called B, the rife of the barometer, for an in- 
creafe of heat from freezing to boiling water, will 
jg 
be — ; and, as it will be lefs for a lefs difference 
5 + 
of heat, therefore, if the number of degrees, 
marked on the thermometer, between freezing and 
boiling water, be called K, and the rife of the 
thermometer from any given point be called H, 
the correfpondent rife of the barometer will be 
B H 
— x j7> by the increafe of heat from the given 
point by the number of degrees H. If the heat, 
inftead of increafing, \Vas to decreafe, then H 
would 
