THERMAL EFFECTS OF FLUIDS IN MOTION. 
329 
With reference to the experiments in Table VII. it may be remarked, that the 
cooling effect must be the excess of that which would have been obtained had the air 
been only resisted by the atmospheric pressure in escaping from the plug, above the 
cooling effect that would be found in an experiment with the temperature of the bath 
and the pressure of the entering air the same as the temperature and pressure of the 
exit air in the actual experiment, and the air issuing at atmospheric pressure. Hence, 
since two or three degrees of difference of temperature in the bath would not sensibly 
alter the cooling effect in any of the experiments on air, the cooling effect in an 
experiment in which the pressure of the exit air is increased, must be sensibly equal 
to the difference of the cooling effects in two of the ordinary experiments, with the 
high pressures the same as those used for the entering and issuing air respectively, 
and the low pressure that of the atmosphere in each case ; a conclusion which is 
verified by the actual results, as the comparison given below shows. 
The results recorded in the foregoing Tables are laid down on Chart No. 3. in which 
the horizontal lines represent the excess of the pressure of the air in the receiver over 
that of the exit air as found by subtracting the fifth from the fourth columns of the 
Tables, and the vertical lines represent the cooling effect in tenths of a degree 
Centigrade. It will be remarked that the line drawn through the points of observa- 
tion is nearly straight, indicating that the cooling effect is, approximately at least, 
proportional to the excess of pressure, being about *018° per pound on the square 
inch of difference of pressure. Or we may arrive at the same conclusion by dividing 
the cooling effect (S) by the difference of pressures (P— P') in the different experi- 
ments. We thus find, from the means shown in the different tables, — 
Table (I.) p _ p , 
= •0170 
(II.) 
•0179 
(III.) 
•0165 
(IV.) 
•0196 
(V.) 
•0177 
(VI.) 
•0172 
(VII.) 
•0174 
Mean 
•0176 
On the Cooling Effects experienced hy Carbonic Acid in passing through a porous Plug. 
The position of the apparatus gave us considerable practical facilities in experi- 
menting with carbonic acid. A fermenting tun 10 feet deep and 8 feet square was 
filled with wort to a depth of 6 feet. After the fermentation had been carried on for 
about forty hours, the gas was found to be produced in sufficient quantity to supply 
the pump for the requisite time. The carbonic acid was conveyed by a gutta-percha 
pipe, and passed through two glass vessels surrounded by ice in order to condense 
the greater portion of vapours. In the succeeding experiment the total quantity of 
liquid so condensed was 300 grains, which having a specific gravity of ’9965, was 
composed of 10 grains of alcohol and 290 grains of water. On analysing a portion 
of the gas during the experiment by passing it through a tube containing chloride of 
zinc, it was found to contain 0733 gr. of water to 100 grs. of carbonic acid. 
2 u 
MDCCCLIV. 
