PROFESSOR W. THOMSON AND MR. J. P. JOULE ON THE 
which it is kept at a distance of 6 inches. The coils are connected by means of a 
coupling joint d, near which a stopcock, e, is placed, in order to let a portion of air 
escape when it is wanted to reduce the pressure. The terminal coil has a flange, 
f to which any required nozle may be attached by means of screw-bolts, ^ear 
the flange, a small pipe, g, is screwed, at the termination of which a calibrated glass 
tube bent (as shown in fig. 4), and partly filled with mercury, is tightly secured. .\ 
stopcock at h, and another in a small branch pipe at i, permit the air at any time to 
be let off, so as to examine the state of the gauge when uninfluenced by any e.xcept 
atmospheric pressure. The branch pipe is also employed in collecting a smal portion 
of air for chemical analysis during each experiment. A pipe, j, is so suspended, that 
by means of india-rubber junctions, a communication can readily be made to convey 
the air issuing from the nozle into the gas-meter, k, which has a capacity of 40 cubic 
feet, and is carefully graduated by calibration. A bent glass tube, /, inserted m the 
top 'of the meter, and containing a little water, indicates the slight difference which 
sometimes exists between the pressure of air in the meter and that of the external 
atmosphere. When required, a wrought-iron pipe, m, 1 inch in diameter, is used to 
convey the elastic fluid from the meter to the desiccating apparatus, and thence to 
the pump so as to circulate through the entire apparatus. 
We have already pointed out the different thermal effects to be anticipated from 
the rushing of air from a single narrow orifice. They are cold, on the one hand, from 
the expenditure of heat in labouring force to communicate rapid motion to the air 
by means of expansion ; and heat, on the other, in consequence of the vis viva of the 
rushing air being reconverted into heat. The two opposite effects nearly neutralize 
each other at 2 or 3 inches distance from the orifice, leaving however a slight pre- 
ponderance of cooling effect; but close to the orifice the variations of temperature 
are excessive, as will be made manifest by the following experiments. 
A thin plate of copper, having a hole of -^th ot an inch diameter, drilled in the 
centre, was bolted to the flange, an india-rubber washer making the joint air-tight. 
At the ordinary velocity of the pump the orifice was sufficient to discharge the iihole 
quantity of air when its pressure arrived at 124 lbs. on the square inch. When 
however lower pressures were tried, the stopcock e was kept partially open. The 
thermometer used was one with a spherical bulb 0’15 of an inch in diameter. 
Holding it as close to the orifice as possible without touching the metal, the folloii- 
ing observations were made at various pressures, the temperature of the water in 
which the coils were immersed being 22° Cent. The air was dried and deprived of 
carbonic acid by passing it, previous to entering the pump, through a vessel 4^ feet 
long and 20 inches diameter, filled with quicklime. 
Total pressure of the air in 
lbs. on the square inch. 
124 
72 
31 
Temperature 
Centigrade. 
8-58 
1 1‘65 
16-2.5 
Depression below 
temperature of bath. 
13-42 
10-35 
5-/5 
