ABS, 
LXXVI. Extract from a Letter dated Cleveland, Ohio, August 
dth, 1904, to Lord Kelvin from Profs. Epwarp W. Morey 
and Dayvon C. MILLER. 
N 1887 Michelson and Morley made an experiment on the 
relative motion of the earth and the luminiferous ether. 
They could detect no relative motion equal to one sixth of 
the earth’s velocity in its orbit. 
_ To explain this result, FitzGerald and Lorentz suggested 
that the stone slab on which the apparatus was constructed 
might have its dimensions changed by its drift through the 
ether. 
There was a remote chance of detecting such an effect by 
repeating the experiment of 1887, but with different materials. 
If the FitzGerald-Lorentz effect exists, it may affect all 
materials to the same amount, independently of the nature of 
the material. But it is also possible that the effect is one 
which depends on the physical properties of the material, so 
that pine may be affected more than sandstone. In this 
ease, if sandstone gives no displacement in an experiment 
like that of 1887, an apparatus supported by pine, which 
would be compressed more than sandstone, would give an 
effect of the sign opposite to that suggested by the original 
simple theory. 
Such an experiment has now been made. We first made 
a structure of pine, which we floated on mercury as in 1887. 
While this structure was new, we obtained good observations 
with it. But after the wood had been affected by steam-heat 
for one season, it was not possible to maintain it in adjustment 
for five minutes. We therefore made a structure in which 
the distance of the mirrors depends on pine rods, but all 
other circumstances depend on steel. Two very stiff steel 
girders cross symmetrically, and are floated on mercury. 
Two holders, each carrying four mirrors, are fixed at the 
ends of two arms of this cross. Two other mirror-holders 
are suspended freely. Pine rods reach from the fixed 
holders to the free holders, and springs maintain stable 
contact. These pine rods are carried in brass tubes which 
constitute a sort of truss; the distance of the two sets of 
mirrors depends solely on these pine rods. 
Observations were made by noting the place of the central 
black fringe on a kind of eyepiece micrometer. They were 
made at sixteen equidistant azimuths, and commonly at a 
rate of one revolution in little more than a minute, readings 
being made to the tenth of a wave-length. ‘Two times were 
