51 
]\Ir Adie on the Sympiesometcr, 
apply a correction on this account. For this purpose the prin- 
cipal or barometric scale m n, is made to slide upon another scale 
0 p, placed either below it or on one side of it, which is divided 
into degrees and parts, so as to represent the change of bulk in 
the gas produced, by a change of temperature under the same 
pressure, and corresponding to the degrees of a common Ther- 
mometer attached to the instrument. 
This scale is constructed in the same manner as the scale of a 
common thermometer, by changing the temperature of the bulb 
while the pressure is the same, and noting the range of the oil 
occasioned by it. 
In using the instrument, observe the temperature by the 
thermometer, and set the index which is upon the sliding Sym- 
piesometer scale, opposite to the degree of temperature upon 
the fixed scale ; and then the height of the oil, as indicated on 
the sliding scale, will be the pressure of the air required. 
When the height of one place above another is to be measur- 
ed by the diminution of the pressure of the atmosphere, ano- 
ther correction is necessary to insure perfect accuracy in all in- 
struments indicating this change, because the pressure of a 
column of air of a given altitude varies according to its humidity, 
or moisture. I have therefore added to the Sympiesometer a 
new Hygrometer, which has already been described in a preceding 
article. See page 32. 
In some of the Sympiesometers which I have made, the scale 
is divided into parts corresponding to the increase in bulk which 
takes place in the gas by the diminished pressure of tlie atmo- 
sphere on ascending a given height, the temperature being 32'^ of 
Fahrenheit. This scale is also formed by experiment, as follows : 
The instrument being placed in the glass-case as before describ- 
ed, increase the density of the inclosed air until it Support a 
column of quicksilver of 31 inches, the temperature being SS*". 
Mark this point zero; thenfrom the logarithmof 31 subtract .0100, 
and find the corresponding number, which is 30.294 ; regulate 
the density of the air to support a column of quicksilver of this 
length; number this point on the scale 100, and divide the 
space into 100 parts ; each part will equal the increase of bulk 
or fall of the oil in the tube by ascending one fathom. In the 
above manner proceed^ by subtracting .0100 lr(.)m the logarithm 
