meafuring Heights with the Barometer. 679 
plifhed without many troublefome alterations in the ap- 
paratus, therefore nothing of the kind was attempted. 
From the circumftances juft now mentioned, it will be 
readily conceived, how much care is neceflary in ope- 
rating with barometers for the meafurement of heights, 
that the vacua be as nearly as poflible compleat; and par- 
ticularly, that no moifture get up into the tube. I now 
proceed to the fourth and laft fet of experiments. 
Having found, from the two firft fets of this clafs, the 
rate of expanfion of a column of quickfilver, in the tube 
of a barometer of the ordinary length, to be progreffive 
and not arithmetical ; and that its maximum, for the 1 8o° 
comprehended between feeezing and boiling, was lefs 
than had been fuppofed; I thought it proper to try, by 
means of artificial cold, whether the condenfation, for the 
3 2 0 below freezing, followed nearly the fame law ? 
For this purpofe the tin veflel, containing the ground 
tube, was rammed quite full of pounded ice and fait, as 
well as the tin ftand holding the iron cittern below. In 
this operation, twelve pounds of ice and four pounds of 
fait were employed, whereby the mean temperature of 
the mixture was reduced to + 4 0 of Fahrenheit. But be- 
fore the eyes of the veflel could be fufficiently freed 
from the compofition, fo as to permit the furface of the 
column to be diftincftly feen and read off; it had rifen to 
5 + 1 4 ° 
