316 Prof. A. J I. Bucherer on tlie 
temperature. H an empirical formula wore found to lit the 
values of the Joule-Thomson effect over a wide range of 
temperature, then we might very well conclude — apart from 
any molecular hypothesis — that this formula was the proper 
one to employ. It is true that measurements of the Joule- 
Thomson effect are far from easy to carry out satisfactorily ; 
still, the difficulties are not insuperable, and there is no 
reason why the success of Joule and Kelvin in this line should 
not he repeated. The measurements of these last-named 
experimenters appear to have been confined within narrow 
limits of temperature, not so much because observations were 
impossible at temperatures outside these limits, as because 
Lord Kelvin imagined he had already discovered the true 
formula for the Joule-Thomson effect. 
The plan here advocated of repeating the Joule-Thomson 
experiments over a wider range of temperature is all the 
more feasible since we have shown that we require only the 
relative values of the Joule-Thomson effect. Thus any 
source of error which multiplies all the Joule-Thomson effects 
by the same factor would be eliminated The errors of ex- 
periment which give rise to ordinary ml wobbling " would 
also be eliminated by the present method. Indeed, the onlv 
sources of error which are liable to affect the final numerical 
results to a sensible degree are those which tend persistently 
to increase or diminish the Joule-Thomson effect at higher 
temperatures as compared with that at lower. Provided 
that such sources of error were either abolished or properly 
allowed for, we could place considerable contidence in the 
final numerical results, and probably succeed in throwing 
great light on a fundamental problem of thermodynamics. 
XXX. On the Principle of Relativity and on the Electro- 
magnetic Mass of the Electron. A Reply to Mr. E. Cun- 
ningham. By A. H. Bucherer, D.Sc, Professor in the 
Bonn University *. 
IX the October number of this Magazine Mr. E. Cunningham 
raises some objections to the theory of relativity as de- 
fined by me in the April number. I wish to say a few words 
in reply in order to show that Mr. Cunningham's remarks are 
due to a misconception on his part of the real meaning and 
bearing of the principle as used by me. 
As appears from my paper, my object has been to find 
a purely phenomenological method of calculating electro- 
magnetic effects, which should harmonize with all the facts 
* Communicated bv the Author. 
