VOL. LXIV.] PHILOSOPHICAL TRANSACTIONS. ' 321 



the frame of the barometer itself, and the other made to be exposed to the open 

 air, to show its degree of heat; which thermometers are to be noted both at the 

 top and bottom of the hill. Lastly, by a great number of experiments made 

 with accurate barometers and thermometers of his own construction, he has 

 deduced a rule for calculating heights of places; the exactness of which he has 

 sufficiently proved by a large table of experiments. But this rule is expressed in 

 French measure, and is adapted either to a thermometer, whose freezing point 

 is O, and that of boiling water 80, or to thermometers of particular scales. It 

 may therefore be useful to reduce M. de Luc's rule to English measure, and to 

 adapt it to the thermometer of Fahrenheit's scale, which is generally used in 

 this country. 



M. de Luc, in the winter season, heated the air of his room to as great 

 degree as he could, and noted the rise of the barometer, owing to the diminu- 

 tion of its density, or specific gravity, by heat ; he also 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 case in 

 this experiment, an increase of heat, from freezing to that of boiling water, will raise 

 the barometer 6 lines, or ^i^ part of the whole. It is easy to see, that when the 

 barometer is higher than 27 French inches, this variation will increase in the 

 same proportion; or will be always -^ of the height of the barometer; there- 

 fore, if the height of the barometer be called b, the rise of the barometer, for 

 an increase of heat from freezing to boiling water, will be — ; and, as it will 



be less for a less difFeience of heat, therefore, if the number of degrees, marked 

 on the thermometer, between freezing and boiling water, he called k, and the 

 rise of the thermometer from any given point be called h, the correspondent 

 rise of the barometer will be — x -, by the increase of heat from the given 

 point by the number of degrees h. If the heat, instead of increasing, was to 

 decrease, then h would signify so many degrees decrease of heat, and the 

 barometer would sink, by — X -. The fixed temperature of heat, to which M. 

 De Luc thought best to reduce his observations of the barometer, is -f of the 

 interval from freezing to boiling water above the former point: and if the ther- 

 mometer was higher than this degree, he subtracted — X - ; if it was lower, 

 he added it to the observed height of the barometer; and thus he obtained the 

 exact height of the barometer, such as it would have been, if the density of 

 its quicksilver had been the same as answers to the fixed degree of temperature. 

 He thus corrected the height of both his barometers (that at the bottom, and 

 that at the top of the hill) for the particular degree of heat, indicated by a ther 

 mometer attached to the barometer, at each station; for it might and would 

 commonly happen, that the degree of heat would be different at the two stations. 

 The heights of the barometers, thus corrected, were what he made use of in 



VOL. XIII. 3 X 



