90 New experimental Researches 
The rule on which the preceding table is formed, may be ex- 
pressed in a manner better fitted to give directly the elastic force 
corresponding to any given temperature moderately distant from 
212°. It becomes also more accurate. 
Let 7 = the mean ratio between 210° and the given tempera- 
ture; 2 = the number of terms (each of 10°) distant from 210°5 
F= the elastic force of steam in inches of mercury. 
Then, Log. of F=Log. 289+ 2. Log. 7; the positive sigh 
being used above, the negative below 210°. 
Or by common arithmetic, multiply or divide 28-9, according 
as the temperature is above or below 210°, by the mean ratio, 
involved to a power denoted by the number of terms. The pro- 
duct or quotient is the tension required. 
Example \st. The temperature is 140°. What is the corre- 
sponding elasticity of the vapour from water heated to that point? 
140° is 7 terms of 10° each under 210°; 1:26 is the mean 
123+1°29 
; and, consequently, r=1:26; n=7. 
Log. 28:9= 146090 
Log. 1:26 x7= (10087 x 7 = —0°70259 
0:75831, which’ is 
the logarithm of = -. e* =: 5732 inches. 
Experiment gives ++ ++ °° 5°77, difference °04, in- 
’ considerable. 
Example 2. What is the tension of steam at the tempera- 
ture of 290°? ' 
ree =1:195 n=5 
Log. 28:9 = 146090 
8 Log. r =8x0077387 = +0:61896 
ratio = 
Log. of 120-02 inches 2.07986 
At 290° by experiment = 120-15. 
Example 3. Temperature 250°. Force of steam in con- 
tact with water? 
pe eee =1°215 nz=4 
Log. 28:9 = 146090 
4 Log. r=4 x 0°08458 = +0°33832 
Log. of 62°98 1-79922 
At 250° Experiment 61-90 
At these high heats, it is very possible that the experiment 
may be in error by one inch, which is the whole difference here. 
About half a degree of Fahrenheit misnoted, would give this de- 
viation. 
Such a correspondence, therefore, of observation with the cal- 
culated 
