627 
of Edinburgh, Session 1881-82. 
if e be - (as in the case of water under the ordinary atmospheric 
pressure between 0° and 4°), for a sudden decrease of pressure there 
will be an increase of temperature ; and lastly, if a body have a 
maximum density point (as water under the ordinary atmospheric 
pressure) or a minimum density point, then just at that temperature 
there is no change of temperature for a small sudden change of 
pressure. If, however, when water at 4° C. is under great pressure, 
we prove that for a sudden change of pressure e is + , we at once 
infer that under such pressure water has not a maximum density at 
that temperature. It is in this way we have proved the lowering 
of the maximum density point of water by pressure, and also been 
able to approximate to the maximum density points under a few 
pressures. 
The apparatus we used consisted of four parts : (1) the apparatus 
for producing the compression, consisting of a strong iron vessel 
(called the small gun) and a powerful force pump ; (2) the gauge ; 
(3) the galvanometer, to measure the changes of temperature follow- 
ing changes of pressure ; and (4) an extra thermo-electric circuit, to 
determine the values in thermometric degrees of the galvanometer 
deflections. 
The first two pieces of apparatus have already been described 
by Professor Tait in the account of his experiments above alluded 
to. 
The changes of temperature which followed changes of pressure 
were measured by thermo-electricity. One junction of a thermo- 
electric circuit of very fine copper and iron wire was placed inside 
the gun, the wires passing out between two discs of leather well 
soaked in oil near the bottom, which could be taken off. The others 
were soldered to thick copper wires in connection with a dead-beat 
reflecting galvanometer, and placed in a vessel of water, which was 
kept at as uniform a temperature as possible, one of them being put 
into a test-tube to prevent the slight voltaic action which disturbed 
us in our earliest experiments. As we had in some of our experi- 
ments to measure extremely small changes of temperature, the 
galvanometer was made very sensitive by the controlling magnets, 
and the deflections read on a scale about 2J metres from the galva- 
nometer. We were enabled in this way to measure as small a 
change of temperature as T C. even with a single thermo-electric 
VOL. xi. 4 D 
