THERMAL EFFECTS OF FLUIDS IN MOTION. 
363 
The observations above tabulated were made at intervals of two or three minutes. 
It will be observed that the cooling effect appeared to be greater at the commence- 
ment than at the termination of the series. This may be attributed in a great 
measure to the drying of the cotton, which was found to contain at least 5 per 
cent, of moisture after exposure to the atmosphere. There was also another source 
of interference with the accuracy of the results owing to a considerable oscillation 
of pressure arising from the action of the pump. We had remarked that when 
the number of strokes of the engine was suddenly reduced from twenty-seven to 
twenty-five per minute, a depression of the thermometer equal to some hundredths 
of a degree Cent, took place, a circumstance evidently owing to the entire mass 
of air in the coils and cotton plugs suffering dilatation without allowing time 
for the escape of the consequent thermal effect. Hence it was found absolutely 
essential to keep the pump working at a perfectly uniform rate. For a similar reason 
it was also most important to prevent the oscillations of pressure due to the action 
of the pump, particularly as it appeared obvious that the heat evolved by the sudden 
increase of pressure, on the admission of a fresh supply of air from the pump, would 
arrive at the thermometer in a larger proportion than the cold produced by the sub- 
sequent gradual dilatation. In fact, on making an experiment in which the air was 
kept at a low pressure, by opening a stopcock provided for the purpose, the oscilla- 
tions of pressure amounting to ^th of the whole, it was found that an apparent 
heating effect, equal to 0°-2 Cent., was produced instead of a small cooling effect. 
It became therefore necessary to obviate the above source of error, and the method 
first employed with that view, was to place a diaphragm of copper with a hole in its 
centre |th of an inch in diameter at the junction between the iron and copper 
pipes. The oscillation being thus reduced, so as to be hardly perceptible, we made 
the following observations. 
Second series of experiments. Atmospheric air dried and deprived of carbonic 
acid by quicklime. Gauge 7375; barometer 30T62= 14755 lbs. pressure per 
square inch; thermometer 19°-3 Cent. 
