of Edinhurgh, Session 1882-83. 
45 
taining 0*06 per cent, and that of air containing 0'2 per cent, of 
olefiant gas. With a rigid reservoir it would he possible to deter- 
mine exactly the constitution of mixtures of air and olefiant gas, 
and of air and water vapour having equal absorptive powers. 
The above experiments were made in Professor Tail’s laboratory 
during the summer vacation of 1882. By the time they had 
advanced thus far my vacation was at an end, and I could not wait 
to get a new reservoir constructed and to complete them. I hope 
Professor Tait may be able to complete them at an early date. 
I must mention with gratitude the enthusiastic assistance ren- 
dered me by Mr Lindsay. His cheerful contempt of failure and 
his ready resource in difficulty contributed largely to whatever of 
success we attained. We are deeply indebted to Professor Crum 
Brown for the use of apparatus necessary for the making of olefiant 
gas. Of course Professor Tait took a most lively interest in our 
results, and we had at all times the benefit of his advice. 
4. Hote on the Compressibility of Water. By Professor Tait 
To test by an independent process the accuracy of the unit 
of my pressure gauge, on which the estimated corrections for the 
“Challenger” deep-sea thermometers depend, it was arranged that 
H.M.S. “Triton” should visit during the autumn a region in which 
soundings of at least a mile and a half could be had. A set of 
manometers, filled with pure water, and recording by the washing 
away of part of a very thin film of silver, were employed. They 
were all previously tested, up to about 2-|- tons weight per square 
inch, in my large apparatus. As I was otherwise engaged. Professor 
Chrystal and Mr Murray kindly undertook the deep-sea observa- 
tions ; and I have recently begun the work of reducing them. 
The first rough reductions seemed to show that my pressure unit’ 
must be somewhere about 20 per cent, too small. As this was the 
all but unanimous verdict of fifteen separate instruments, the sur- 
vivors of two dozen sent out, I immediately repeated the test of my 
unit by means of Amagat’s observed values of the volume of air at 
very high pressures. The result was to confirm, within 1 per cent., 
the accuracy of the former estimate of the unit of my gauge. I then 
had the manometers resilvered, and again tested in the compression 
