measuring Heights with the Barometer. 657 
the quicksilver is exactly fix French lines, or .532875 
decimal parts of an Englifh inch. But it is to be ob- 
ferved, that the barometer flood then at 28.77525 ; 
whereas, if it had flood at 30 inches, it would have been 
• 555556, becaufe the expan lion is in proportion to the 
length of the column. Farther, the interval between the 
freezing and boiling -points in all thermometers, varies 
with the height of the barometer, or weight of the atmo- 
fphere; and it is the cuftom in England to make thermo- 
meters when the barometer Hands at 30 inches; that is 
to fay, 1 . 2 2 5 or 1 3 . 8 Fi'ench lines, higher than when Mr, 
de luc’s boiling point was fixed : and fince from his ex- 
periments it appears, that each line of additional height 
in the barometer, raifes the boiling point TJ E- 4 -th part of 
the interval between that and freezing, it follows that 
ttj4 = 0.158 x 13.8= 2 0 . 2, will denote the number of 
degrees, that Mr. de luc’s boiling point is lower than 
- that of Englifh thermometers, which reduces it to 209.8 
of Fahrenheit, and makes the interval between freez- 
ing and boiling only 177.8 degrees. Hence the expan-, 
fion .555556, formerly found, muft be increafed in the 
proportion of 177.8 to 180, which gives for the total 
.5624297 or .56243, on a difference of temperature of 
1 8o°. Thus the expanfion for each degree, fuppofing it 
Vol. LXVII. 4 Q to 
