PRESIDENT’S ADDRESS. 21 
alone has ever yet been observed: there are always curious com- 
pensating effects which just cancel out the movement-terms and 
destroy or effectively mask any phenomenon that might otherwise 
be expected. When matter moves past matter observation can be 
made; but, even so, no consequent locomotion of ether, outside the 
actually moving particles, can be detected. 
It is sometimes urged that rotation is a kind of absolute motion 
that can be detected, even in isolation. It can so be detected, as 
Newton pointed out; but in cases of rotation matter on one side the 
axis is Moving in the opposite direction to matter on the other side 
of the axis; hence rotation involves relative material motion, and 
therefore can be observed. 
To detect motion through ether we must use an etherial process. 
We may use radiation, and try to compare the speeds of light along 
or across the motion; or we might try to measure the speed, first 
with the motion, and then against it. But how are we to make the 
comparison? If the time of emission from a distant source is given 
by a distant clock, that clock must be observed through a telescope, 
that is by a beam of light; which is plainly a compensating process. 
Or the light from a neighbouring source can be sent back to us by a 
distant mirror; when again there will be compensation. Or the start- 
ing of light from a distant terrestrial source may be telegraphed to us, 
either with a wire or without; but it is the ether that conveys the 
message in either case, so again there will be compensation. 
Electricity, Magnetism, and Light, are all effects of the ether. 
Use Cohesion, then; have a rod stretching from one place to 
another, and measure that. But cohesion is transmitted by the ether 
too, if, as believed, it is the universal binding medium. Compensa- 
tion is likely; compensation can, on the electrical theory of matter, be 
predicted. 
Use some action not dependent on Ether, then. Very well, where 
shall we find it? 
To illustrate the difficulty I will quote a sentence from Sir Joseph 
Larmor’s paper before the International Congress of Mathematicians 
at Cambridge last year :— 
“If it is correct to say with Maxwell that all radiation is an 
electrodynamic phenomenon, it is equally correct to say with 
him that all electrodynamic relations between material bodies are 
established by the operation, on the molecules of those bodies, 
of fields of force which are propagated in free space as radiation 
and in accordance with the laws of radiation, from one body to 
the other.’ 
The fact is we are living in an epoch of some very comprehensive 
