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XXV. On the Thermal Effects of Fluids in Motion . — Part IV. 
P. Joule, LL.D., F.B.S. &c., and Professor W. Thomson, A.M., LL.D., F.B.S. &c. 
Received June 19, — Read June 19, 1862. 
In the Second Part of these researches we have given the results of our experiments on 
the difference between the temperatures of an elastic fluid on the high- and low-pressure 
sides of a porous plug through which it was transmitted. The gases employed were 
atmospheric air and carbonic acid. With the former, 0°'0176 of cooling effect was 
observed for each pound per square inch of difference of pressure, the temperature on 
the high-pressure side being 17°T25. With the latter gas, 0°- 08 3 3 of cooling effect was 
produced per lb. of difference of pressure, the temperature on the high-pressure side 
being 12°-844. 
It was also shown that in each of the above gases the difference of the temperatures 
on the opposite sides of the porous plug is sensibly proportional to the difference of the 
pressures. 
An attempt was also made to ascertain the cooling effect when elastic fluids of high 
temperature were employed ; and it was satisfactorily sho'wn that in this case a consider- 
able diminution of the effect took place. Thus, in air at 91°-58, the effect was only 
0°’014; and in carbonic acid at 91°'52, it was 0°‘0474. 
In the experiments at high temperatures there appeared to be some grounds for 
suspecting that the apparent cooling effect was too high ; for the quantity of trans- 
mitted air was very considerable, and its temperature possibly had not arrived accurately 
at that of the bath by the time it reached the porous plug. 
The obvious way to get rid of all uncertainty on this head was to increase the length 
of the coil of pipes. Hence in the following experiments the total length of 2-inch 
copper pipe immersed in the bath was 60 feet instead of 35, as in the former series. 
The volume of air transmitted in a given time was also considerably less. There could 
therefore be no doubt that the temperature of the air on its arrival at the plug was 
sensibly the same as that of the bath. 
The nozle employed in the former series of experiments was of box-wood, — the space 
occupied by cotton-wool, or other porous material, being 2-72 inches long and an inch 
and a half in diameter. The box-wood was protected from the water of the bath by 
being enveloped by a tin can fllled with cotton-wool. This was unquestionably in most 
respects the best arrangement for obtaining accurate results ; but it was found necessary 
to make each experiment last one hour or more before we could confidently depend on 
the thermal effect. The oscillations of temperature which took place during the first 
MDCCCLXII. 4 K 
