NEWTONIAN CONSTANT OF GRAVITATION. 115}. 
in all the experiments made up to the present. The dummy was provided partly to 
absorb radiation from the flame and so to protect the working scale from heating, 
but mainly so that I should have a glass plate ready to divide myself without loss of 
time in case of accident to the working scale. This happily has not occurred. 
I calibrated this scale by reference to an American steel scale divided into 50ths 
of an inch, the uniformity of which I had previously tested. This steel scale became for 
the purpose of the angular measurements the standard to which everything was 
referred. For this purpose its absolute value is of no consequence ; all that matters 
is its uniformity. A long board was supported so that its upper surface was every- 
where level. Sheet lead strips were rolled until they were just thicker than the steel 
scale and a double row were laid upon the level board. The glass scale was made to 
rest upon these with its face downwards and the steel scale was slipped underneath, 
so that the glass and steel divisions should be superposed. An erecting eyepiece was 
placed on a stand above the glass and was used as a reading microscope. Every tenth 
division was observed, and the 480 corrections were entered in a book, and were 
also plotted out on an enlarged scale, so that an error of 3$9 inch should be repre- 
sented by 39 inch. From the irregular curve drawn through all the points the 
calibration error of every scale reading was afterwards ascertained. In order to 
determine the circular error, the true distance in scale divisions between the mirror 
and the scale was measured according to the plan to be described on p. 17. A. large 
number of values of the circular correction were calculated from the expansion 
for tan~' « and tabulated in terms of scale divisions. It was necessary to include 
the term + = as at the ends of the scale this amounted to half a division, while at 
1800 divisions on either side of the perpendicular reading it was one-tenth of a 
division. The perpendicular reading 22,600 being invariable, these corrections were 
plotted on the same sheet as the calibration errors, thus the two corrections could 
be taken out simultaneously for every reading which was thus converted into the 
reading that would have been obtained if every division subtended the same angle at 
the mirror. In this way the time and labour that are ordinarily required in finding 
the angles corresponding to scale divisions and in correcting for calibration are 
reduced to a few seconds for each, and error is almost impossible. 
The Overhead Pullies. 
The overhead wheels are eight in number, and are all of the same size. Five are 
over the instrument, and three close to the west wall. As already stated the edge 
of the middle one, which has a round groove in it, is exactly over the centre of the 
apparatus. Those on either side have flat-bottomed grooves, and they can be placed 
either 6 or 4 inches apart, according as 44 or 24-inch balls are to be used. Outside 
these, and the same distance apart as the screwed pillars R Rin the lid, are two round 
