C 2 3 6 ] 
correction, I am now confidering, o ; but only, that, 
its temperature, whatever it be, be the fame at both 
ftations. If the temperature hath been the fame, 
the fpecijic gravity hath been the fame; and if the 
fpecijic gravity hath been the lame, the length of 
the columns of quickfilver have been as the forces 
by which they were fuflained, whatever the com- 
mon fpecific gravity may have been. Columns of 
water, fuffained in evacuated tubes, of Efficient 
length, would be proportional to columns of quick- 
ftlver in the barometer, at the fame times and places, 
provided the temperature of the two water columns 
were the fame, and that of the two mercurial co- 
lumns the fame; and, confequently, the difference 
of the logarithms of the water columns would be 
precifely the fame as of 'the mercurial columns. 
For the water columns, and the mercurial columns, 
are only expriffions of the fame abfolute magni- 
tudes, the forces by which they are both fuffained, 
in parts of different fcales. But if the temperature 
of the quickfilver, or of the water, be different at 
the fame time, at different ffations; then, though 
we compare the water columns with each other, 
and the mercurial columns with each other, {fill we 
compare different things, though we call them by 
the lame name. We compare fluids of different 
ipecific gravities ; that is, we meafure the preflure 
of the air at one place, in parts of one fcale, and at 
the other place, in parts of another. The error 
is of the fame kind, as if one fhould attempt to de- 
termine the proportion between two parts of a 
building, by meafuring one with a Paris foot-rule, 
and the other with an Englifh foot-rule, without 
attending 
