in order to afcertain the height of Mountains . 567 
each degree of the thermometer, commencing from the 
freezing point, =0,00262 inch on a column of 30 
inches of the barometer, if the glafs had fuffered no ex- 
pan fion during the experiment. This, however, has 
been found to be with 1 8o° of heat = ~ in folidity 
(viz. the cube of its longitudinal expanfion) and 
_ — x— — = — - — =0,0004.2 inch, for the effedt of the 
expanfion of the glafs for i° upon a column of 30 
inches ; this added to the quantity before found, which 
was only the excefs of the greater expanfion above the 
lefs, gives for the true equation for each degree 0,00304 
inch when the barometer ftands at 30 inches (c> . Mr. de 
luc’s correction in this cafe was 0,00312; a difference 
fo fmall that I fhall take no notice of it as to its influence 
upon our obfervations. It may deferve a remark here, 
that this equation rigoroufly taken is variable according 
to the height of the thermometer; for i°, which at 
(c) It lias been fufpe&ed, and I believe will appear from very good obferva- 
lions, which however I never made myfelf, that the expanfion of quicklilver in 
the barometer is not direCtly as the heat fhewn by the thermometer, but in a 
ratio lomething different, owing to fame of the quicklilver being converted 
into an elaftic vapour in the vacuum that takes place at the top of the Torri- 
cellian tube, which preffesupon the column of quickfilver, and thus counteracts 
in a fmall degree the expanfion from heat. It does not, however, appear to be 
a conliderable quantity, not amounting to above one fixteenth of the whole 
cxpanlion in a range of 40° of temperature; I Ihall therefore venture to con- 
sider this equation as truly uniform^ lines the error on ten thoufand feet would 
mot amount to five. 
freezing 
