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XV. On the Thermal Effects of Fluids in Motion. — Part II. 
By J. P. Joule, F.R.S. and Professor W. Thomson, M.A . , F.R.S. 
Received June 15, — Read June 15, 1854. 
IN the last experiment related in our former paper*, in which a low pressure of air 
was employed, a considerable variation of the cooling- effect was observed, which it 
was necessary to account for in order to ascertain its influence on the results. We 
therefore continued the experiments at low pressures, trying the various arrange- 
ments which might be supposed to exercise influence over the phenomena. We had 
already interposed a plug of cotton wool between the iron and copper pipes, which 
was found to have the very important effect of equalizing the pressure, besides 
stopping any solid or liquid particles driven from the pump, and which has therefore 
been retained in all the subsequent experiments. Another improvement was now 
effected by introducing a nozle constructed of boxwood, instead of the brass one 
previously used. This nozle is represented by fig. 1. Plate IV., in which a a is a brass 
casting which bolts upon the terminal flange of the copper piping, b h is a turned 
piece of boxwood screwing into the above, having two ledges for the reception of per- 
forated brass plates, the upper plate being secured in its place by the turned boxwood 
c c, which is screwed into the top of the first piece. The space enclosed by the per- 
forated plates is 2‘72 inches long and an inch and a half in diameter, and being filled 
with cotton, silk, or other material more or less compressed, presents as much resistance 
to the passage of the air as may be desired. A tin can d , filled with cotton wool, 
and screwing to the brass casting, serves to keep the water of the bath from coming 
in contact with the boxwood nozle. 
In the following experiments, made in order to ascertain the variations in the 
cooling effect above referred to, the nozle was filled with 382 grs. of cotton wool, 
which was sufficient to keep up a pressure of about 34 lbs. on the inch in the tubes, 
when the pump was working at the ordinary rate. By opening the stopcock in the 
main pipe this pressure could be further reduced to about 22 lbs. by diminishing the 
quantity of air arriving at the nozle. By shutting and opening the stopcock we had 
therefore the means of producing a temporary variation of pressure, and of investi- 
gating its effect on the temperature of the air issuing from the nozle. In the first 
experiments the stopcock was kept open for a length of time, until the temperature 
of the rushing air became pretty constant ; it was then shut for a period of 3f, 7^, 
15, 30 or 60 seconds, then reopened. The oscillations of temperature thus pro- 
* Philosophical Transactions (June), 1853, Part III. 
2 T 
MDCCCLIV. 
