THERMAL EFFECTS OF FLUIDS IN MOTION. 
361 
The heating effect was exhibited as follows: — The bulb of the 
thermometer was inserted into a piece of conical gutta percha pipe 
in such a manner that an extremely narrow passage was allowed 
between the interior surface of the pipe and the bulb. Thus armed, 
the thermometer was held, as represented by fig. 5, at half an inch 
distance from the orifice, when the following results were ob- 
tained : — 
Fig. 5. 
Total pressure of the air in Temperature 
lbs. on the square inch. Centigrade. 
124 4575 
71 39-23 
31 26-2 
Elevation above 
temperature of bath. 
23/5 
17-23 
4-20 
It must be remarked, that the above recorded thermal effects are not to be taken 
as representing the maximum results to be derived from the rushing air at the pres- 
sures named. The determination of these, in the form of experiment above given, 
is prevented by several circumstances. In particular it must be observed, that the 
cooling effects must have been reduced in consequence of the heat evolved by the 
friction of the rushing air against the bulb of the thermometer. The heating effects, 
resulting as they do from the absorption and conversion into beat of the vis viva of 
the rushing air, depend very much upon the narrowness of the space between the 
thermometer and gutta percha pipe. We intend further on to return to this subject, 
but in the mean time will mention three forms of experiment whereby the heating 
effect is very strikingly and instructively exhibited. 
Exj)eriment 1. — The finger and thumb are brought over the orifice, 
as represented in fig. 6, so that by gradually closing them the stream 
of air is pinched. It is found that the effort to close the finger and 
thumb is opposed by considerable force, which increases with the pres- 
sure applied. At the same time a strong tremulous motion is felt and 
a shrill noise is heard, whilst the heat produced in five or six seconds necessitates the 
termination of the experiment. 
Experiment 2. — Fig. 7- The finger is placed over the orifice and 
pressed until a thin stratum of air escapes between the copper-plate 
and the finger. In this case the burning heat of the rushing air is 
equally remarkable in spite of the proximity of the finger to the cold 
metal. 
Experiment 3. — Fig. 8. A piece of thick india-rubber is pressed by 
the finger over the narrow orifice so as to allow a thin stream of air to 
rush between the india-rubber and the plate of copper. In this case 
the india-rubber is speedily raised to a temperature which prevents its 
being handled comfortably. 
Fig. 7. 
Fig. 6. 
3 B 2 
