of Edinburgh, Session 1884-85. 
313 
from experiments made from time to time to determine the deflec- 
tion arising from known changes of temperature of the outer junction. 
The galvanometer was delicate enough to enable us to measure 
differences of temperature amounting to less than -01 degree 
centigrade, and yet not so sensitive as to be seriously affected by 
the traffic in the street without. Each result recorded below is the 
mean of at least five experiments ; in the case of pressures of about 
two tons each result is usually the mean of ten. 22 '5 divisions of 
the pressure gauge correspond to a pressure of one ton on th'e 
square inch, or about 150 atmospheres. 
Each day, with the exception of those on which we found the 
exact value of galvanometer readings by observing the result of 
known changes of temperature of the outer junction, it was our 
habit to take a series of observations for water alone, by means of 
which we could verify the previously ascertained equivalent of 
galvanometer readings. 
It was generally observed that the fall of temperature on relaxa- 
tion of pressure was greater than the corresponding rise on applying 
pressure, owing to some extent to the fact that pressure could be 
more suddenly relieved than applied. In the case of water, we 
observed, as the mean result of many experiments, that the two 
differed in the proportion of about 14 to 13. Thus, on applying a 
pressure of two tons on the square inch, we obtained a deflection 
of 1*3, equivalent to 0 o, 31 C.j while on releasing pressure the 
deflection was 1*4, equivalent to 0°*33 C. This at a temperature of 
16° C., at which, by Sir W. Thomson’s formula, the rise of tempera- 
ture should be 0 o, 35 C. 
Some reduction thus seems necessary of the results given in the 
tables below, though not in the same proportion, since con- 
vection currents present in the considerably large quantity of water 
which filled the gun, would be wanting in the solids and the com- 
paratively small quantities of liquids used. 
To furnish an idea of the general agreement in the results, we 
give the full data from one experiment, taken at random ; namely, 
the effect produced in vulcanite by changes of pressure to the 
extent of about two tons on the square inch. 
