250 
DR. J. H. POYNTING AND MR. P. L. GRAY IN SEARCH OP A 
The fine hand-adjustment consisted of a small wooden plunger working in a tube 
connected with one of the mercury-cups ; by means of a screw the plunger could be 
raised or lowered, and the level of the mercury in the cup varied accordingly. 
If the revolving sphere was found to be gaining or losing, it was quite easy to 
bring it “ up to time” again by working the screw of the plunger. 
The last of the train of driving-wheels was fixed on the tube supporting the 
larger sphere; its rim was divided into equal parts by numbered marks, the use of 
which will be explained directly. There were 20 numbered marks, at 18° interval; 
of these only 10 alternate ones were used for the quicker rotation, while the whole 
20 were used for the slower speed. 
The Observations. 
Two observers were required, one at the telescope to note the position of the 
smaller sphere, the other to regulate the speed of rotation of the larger sphere, and 
to notify when readings were to be taken by the first observer. The motion having- 
been started, and brought to about the right speed, a time-table was rapidly prepared, 
showing the times, on the chronometer used, at which each of the numbered marks 
above mentioned should pass a fixed mark throughout the whole set of observations for 
one occasion. A signal was given at each passage of a mark past the fixed point, the 
observer at the telescope putting down the simultaneous scale-reading in a manner 
which will be understood from Table I., which may serve as a typical record. It does 
not appear to be necessary to give the full details in other cases. If the motion did not 
keep to the time-table, it was easily corrected by the hand adjustment already described. 
Every reading in the same column is taken at the same phase in the rotation of 
the larger sphere, and therefore the mean readings of the columns should preserve 
any periodicity in the motion of the smaller sphere equal to that of the larger sphere, 
and more or less eliminate all others. These mean readings are given at the foot of 
Table I., and appear to indicate a slight periodic vibration, but this might be due to 
a want of symmetry in the larger sphere and its attachments about its axis of rota¬ 
tion, since the system supporting the smaller sphere and mirror was necessarily not 
symmetrical. The observations for each couple were on this account divided into 
two sets : for the semicircular couple the larger sphere was in the second set turned 
through j 80° about a vertical axis from its position in the first set; for the quadrantal 
couple the rotation was 90°. For the final results the means of the results of the two 
sets were taken, in each case after the second set had been advanced by an amount 
corresponding to the change of position of the sphere. 
Table II. contains all the mean results obtained in the same way as the figures at 
the foot of Table I., the greatest range being given in the last column as an indication 
of the magnitude of the disturbances. 
In Table III. are given the means for each azimuth of the larger sphere in its 
support, the B and D series being advanced as mentioned above. 
