126 . Mr. J. D. Hamilton Dickson on the 
by calling the attention of oceanographers to the proper 
oscillation of large bays. 
In conclusion we wish to express our best thanks to 
Professor H. Nagaoka, under whose supervision the present 
work has been carried out. Equal thanks are due to Professor 
F. Omori, who favoured us with many records of sea-waves. 
Lastly, our cordial thanks are due to Dr. Y. Yoshida, 
Dr. N. Watanabe, Dr. S. Iwamoto, Dr. Y. Inouye, Dr. J. 
Fukuda, and Dr. Hirata, who have been zealous cooperators 
in the course of the observations. 
IX. On the Joule-Kelvin Inversion Temperature, and 
Olszewski s Experiment. By J. D. Hamilton Dickson, 
M.A., Fellow of Peterhouse, Cambridge ; F.R.S.E* 
[Plate VIII.] 
THE condensation of gases and the means of reaching 
low temperatures have, within the last dozen years, 
brought into prominence what is called the Joule-Kelvin 
inversion-temperature. Whether Mayer's hypothesis would 
be established as a fact, led to Joule's experiments on the 
expansion of a gas at high pressure. These experiments 
seemed to establish this hypothesis as true ; but the more 
ingenious method of experimenting suggested by Kelvin, 
in which sudden expansion through a small aperture from a 
high pressure to a low one was replaced by continued 
expansion through a porous plug between two pressures 
differing but little from each other, brought out the fact 
that in general such expansions produced cooling, and thus 
disproved Mayer's hypothesis. 
This result was at first found to be true of all gases ; but 
further examination of hydrogen showed that heating was 
experienced in its case. The theory of the porous-plug 
experiment was given at the time by Kelvin, and the prime 
use made of it then was to determine how far "absolute 
temperature" differed from temperature given by an air- 
thermometer. The most careful precautions were taken, 
by means of concomitant correction experiments, to determine 
exactly the conditions of the porous-plug experiment ; and it 
may be said today, that after a lapse of nearly 60 years 
no improvement has been made in them. 
* Communicated by the Author. 
