THERMAL EFFECTS OF FLUIDS IN MOTION. 
325 
elevations of temperature shown in the second set of diagrams, which correspond to 
cases when the pressure was temporarily or permanently diminished , are not, so far 
as we see, explained by the causes we have mentioned, and the circumstances of these 
cases require further examination. 
When we had thus examined the causes of the fluctuations of temperature in the 
issuing air, the precautions to prevent their injurious effect upon the accuracy of the 
determinations of the cooling effect in the passage of air through the porous plug 
became evident. These were simply to render the action of the pump as uniform 
as possible, and to commence the record of observations only after one hour and a 
half or two hours had elapsed from the starting of the pump. The system then 
adopted was to observe the thermometers in the bath and stream of air, and the 
pressure-gauge every two minutes or minute and a half ; the means of which observa- 
tions are recorded in the columns of the Tables. In some instances the air previous 
to passing into the pump was transmitted through a cylinder which had been filled 
with quicklime. But since by previous use its power of absorbing water had been 
considerably deteriorated, a portion of the air was always transmitted through a 
Liebig tube containing asbestos moistened with sulphuric acid or chloride of zinc. 
The influence of a small quantity of moisture in the air is trifling, but will hereafter 
be examined. That of the carbonic acid contained by the atmosphere was, as will 
appear in the sequel, quite inappreciable. It will be proper to observe that the 
thermometers by which the temperature of the bath and issuing air was ascertained, 
were repeatedly compared together to avoid any error which might arise from the 
alteration of their fixed points from time to time. 
Table I. — Experiments with a plug consisting of 191 grains of cotton wool. 
1 . 
2. 
3. 
4. 
5. 
6. 
7- 
8. 
Number of obser- 
vations from 
which the results 
in Columns 
4, 6, and 7, 
are obtained. 
Cubic inches 
passed through 
the nozle 
per minute. 
Water in 
100 grains 
of air, in 
grains. 
Pressure in lbs. 
on the square 
inch. 
Atmospheric 
pressure. 
Temperature of 
the bath. 
Temperature 
of the issuing 
air. 
Cooling 
effect 
in Cent, 
degrees. 
20 
10822 
0-51 
21-326 
14-400 
20-295 
20-201 
0-094 
20 
10998 
0-30 
21-239 
14-252 
16-740 
16-615 
0-125 
10 
Not observed. 
0-56 
20-446 
14-609 
17-738 
17-622 
0-116 
10 
10769 
0-66 
20-910 
14-772 
16-039 
15-924 
0-115 
10 
10769 
0-66 
20-934 
14-775 
16-065 
15-967 
0-098 
10 
10769 
0-66 
20-995 
14-779 
16-084 
15-984 
0-100 
10 
10769 
0-66 
20-933 
14-782 
16-081 
15-974 
0-107 
Mean 
0-57 
20-969 
14-624 
17-006 
16-898 
0-108 
In each, excepting the first of the seven experiments above recorded, the air was 
passed through the quicklime cylinder. 
In the next experiments the nozle was filled with 382 grains of cotton wool. The 
