of Edinburgh, Session 1880-81. 
51 
be formed beyotid ; the true focal distance for parallel rays being 
the harmonic mean between the two. Hence, if we modify the 
position until the object and its image coincide, we shall have the 
exact position of the principal focus ; always, however, subject to 
the condition that the mirror be quite flat. 
A convenient plan is to draw two strong lines crossing each other 
on a piece of stiff paper, repeating them exactly on the other side, 
and then to cut the paper in two through the centre of the cross ; 
using only the one part. The exact completion of the half-cross 
by its image is a severe test of the adjustment, and the motion 
of the eye across the field at once detects any parallax. A hand 
eye-glass of say one inch in focal length should be used in the 
examination. 
7. Note on the Temperature Changes due to Compression. 
By Professor Tait. 
The author described the results of a number of experiments, made 
during the examination of the “ Challenger ” Deep-Sea Thermo- 
meters, with the view of testing, at pressures of 3 tons weight per 
square inch and upwards, Thomson’s formula for the heat developed 
by compression (Proc. Eoy. Soc., 1857, p. 568). 
When, for instance, the bulb of one of the thermometers was 
surrounded by a shell of lard upwards of half an inch thick, the 
total effect produced by a pressure of 3J tons weight was 5° P. ; 
while for the same pressure, without the lard, the effect was only 
l 0- 8 F. The temperature of the water in the compression apparatus 
was 43° F., so that the temperature effect due to the compression of 
water was less than 0°*2 F. In obtaining this number it was 
assumed from Kopp’s experiments that the coefficient of expansion 
of water at a temperature t° C., near its maximum density point 
1 — 4 
(roughly, 4° C.), is about ^ • Hence the effect due to the com- 
pression of the lard was 3°-4 F., or about 1° F. per ton weight. 
This is subject to corrections (which will increase its value) depending 
on the heat developed by friction in the pump and in the narrow 
connecting tubes, and on another cause not yet fully ascertained. 
