324 
MR. J. P. JOULE AND PROFESSOR THOMSON ON THE 
quarter of a minute ; but it appears to have averaged more nearly one-third of a 
minute in the varying circumstances of the actual experiments, since our observa- 
tions (as may be partially judged from the preceding charts) showed us with very 
remarkable sharpness, in each case about twenty seconds after the shutting or open- 
ing of the stopcock, the commencement of the heating or cooling effect on the issuing 
stream, due to the sudden compression or rarefaction instantaneously produced in 
the air on the other side of the plug. 
The entering air will, very soon after its pressure ceases to vary, be reduced to the 
temperature of the bath by the excellent conducting action of the spiral copper pipe 
through which it passes ; and, consequently, twenty seconds or so later, the issuing 
stream can experience no further fluctuations in temperature except by the agency 
depending on the third cause. 
That the third cause may produce very considerable effects is obvious, when we 
think how great the variations of temperature must be to which the surfaces of the 
solid materials in the neighbourhood of the plug on the high-pressure side are 
subjected during the sudden changes of pressure : and that the heat consequently 
taken in or emitted by these bodies may influence the issuing stream perceptibly for 
a quarter or a half hour after the changes of pressure from which it originated have 
ceased, is quite intelligible on account of the slowness of conduction of heat through 
the wood and metals, when we take into account the actual dimensions of the parts of 
the apparatus round the plug. It is not easy, however, to explain all the fluctuations 
of temperature which have been observed after the pressure had become constant in 
the different cases. Those shown in the first set of diagrams are just such as might 
be expected from the alternate heating and cooling which the solids must have expe- 
rienced at their surfaces on the high-pressure side, and which must be conducted 
through so as to affect the issuing stream after a considerable time ; but the great 
variation as at the entrance, only later in time by an interval equal to — We conclude that the variations 
of temperature in the issuing stream due to the second cause alone, in the actual circumstances, are equal and 
crSa 
similar to those of the air entering the plug, but later in time by — — In this expression, the numerator, aSa, 
6 
denotes simply the thermal capacity of the whole plug. The plug, in the actual experiments, having consisted 
of 382 grains of cotton, of which the thermal capacity is about 191 times that of a grain of water, and (when 
the stopcock was closed) the air having been pumped through at the rate, per second, of 50 grains, of 
which the capacity is twelve times that of a grain of water, the value of — must have been seconds, or 
about a quarter of a minute. When the stopcock was open, an unknown quantity of air escaped through it, 
and therefore the value of must have been somewhat greater. The variation which the value of 8 must 
have experienced when the stopcock was opened or closed in the course of an experiment, or even merely in 
consequence of the change of pressure following the initial opening or closing of the stopcock, makes the 
circumstances not such as in any of the cases to correspond rigorously to the preceding solution ; which, 
notwithstanding, represents the general nature of the convective effect nearly enough for the explanation in 
the text. 
