IGO 
PROFESSOR OLIVER LODGE ON THE ABSENCE OF 
was in immediate contact with the optical frame. The girders at first rested on 
the same gallows as supported the frame (as shown in the figaire, page 767, just 
referred to), but this was ultimately found to be bad, because of a torque receiv'ed 
from the whirling air and transmitted to these piers, which conveyed some trace 
of it to the fi'arae. So while the frame was still supported on its independent piers, 
and the whirling machine was still clamped to its massive stone altar on the rock, 
the drum which received and screened the blast was now separately supported by 
special uprights from the floor (on which people did not walk during an observation), 
and this, on the whole, was an improvement. Any torque effect, however minute, 
being of a reversilfie character, was peculiarly dangerous, for it might easily have 
been mistaken for a result of the kind that was being looked for. 
This memoir shall be abbreviated by the omission of all the careful sets of readings 
taken during this period, a record which occupies seventy pages of the laboratory 
note book ; for it must be admitted that, although representing a good deal of work, 
they fail ultimately to show the air effect; and this probably for the reason that any 
effect of that magnitude would be certainly masked by the residual slight disturbing 
causes present. 
The only thing I will record is a plotting of one of the larger spurious shifts 
(obtained before the foundation was inspected and altered) to illustrate its typical 
lagging character. The dots in this case represent individual readings, not averages 
of setting, and they incidentally show the kind of setting which is possible at high 
speeds through all the cover-glasses, with the light three times round, and when the 
steadiness was by no means perfect. The process was as follows:— 
The micrometer wires were set, the single vertical wire in the middle of the middle 
band, and the X wire on the yellow of the first band to the left; or else vice versa. 
Both wires were read, at gradually increasing, and then at decreasing speeds, and 
the results plotted on the right-hand side of the two diagrams (figs. 5 and 6), so as 
to show (a) the shift of the middle band due to strains and slight communicated 
tremors, (h) the change in the scale of wave-length due to concertina action. Then 
the brushes of the dynamo were reversed, and another spin taken in the opposite 
direction, and then the readings taken which are plotted on the left-hand side of the 
two diagrams. The total maximum shift was about :|;th of a band on this occasion. 
The following averages of a set of readings taken in July, 1893, may also be 
quoted :— 
With disks stationary, tlie middle band read . , 94 4 
Disks revolving 3000 a minnte, the middle band 
read.94'4 
Disks stationary again, the middle band read . 95‘6 
Disks revolving 3000 a minnte in the opposite 
direction, the middle band j-ead.95-5 
Disks stationary again, the middle band read 92‘3 
each division being .V-th of a n'ave-lengtli. 
O O i o 
„ A-tli 
—til 
J 9 0 U 95 
