NEWTONIAN CONSTANT OF GRAVITATION. 17 
The Steel Tape and its Accessories. 
In order to make an accurate determination of the optical distance between the 
reflecting surface of the mirror and the foot of the perpendicular upon the scale, I 
have prepared a steel tape to lie upon the beams L, and L, already described, and 
two sliders, one carrying an erecting eyepiece or low power microscope, and the other 
a sliding brass rod. 
The steel tape is one of ordinary construction, half an inch wide, and divided on 
one side in millims. and on the other in inches and eighths. As the lines on this, as 
is necessary with etched tapes, are thick and raised above the general surface, I 
engraved fine lines on the divisions—2 inch; 7 ft. 4 in.; 14 ft. 6 in.; 21 ft. 8 in. ; 
and 21 ft. 9in. After removing the lead slips which had supported the glass scale 
while it was being calibrated, I laid this scale face downwards on the steel tape, 
setting 0 of the glass scale upon the first engraved line at 2 inch. The reading in scale 
divisions of the fine line at 7 ft. 4 in. was then observed to be 4302°5. The tape was 
drawn back until 0 of the glass scale was over 7 ft. 4 in., and the reading taken for 
14 ft. 6 in.; this was 4302°85. The readings for 21 ft. 8 in. and 21 ft. 9 in., taken 
in the same manner, were 4302°5 and 4352°5. The temperature was 19°75 C. The 
calibration correction for the division 4302 of the glass scale is 3°0. Hence the 
distance in corrected scale divisions from the engraved lines at 2 inch and 21 ft. 8 in. 
at 19°75 C. is 12898°85. The glass scale was calibrated in terms of the steel 
standard at 14°°5 C. ; it had, therefore, relatively contracted at the higher tempera- 
ture. Taking -000002 as the differential coefficient of expansion, the distance 
between the engraved line becomes 12898°98 in terms of the divisions of the 
standard steel scale at any temperature. The sliders have bases made of plate glass, 
on each of which is an engraved cross line. One carries on two V’s the low power 
microscope, and this, after the tape is placed in position, is arranged with its cross- 
line over the engraved line on the 2 inch division. The microscope is then made to 
slide in its V’s until a small cross engraved at the centre of the back of the freely 
suspended mirror is seen through the front window sharply in focus. The microscope 
is then clamped to its V’s, and the slides moved out of position and again set several 
times, the relative position of the engraved lines being noted. If these are systemati- 
cally on one side, the microscope is shifted in its V’s until repeated settings bring the 
engraved lines together. At the other end of the tape a corresponding slider is 
_ placed with its engraved cross-line over one of the engraved lines at 21 ft. 8 in. or 
21 ft. 9 in., and the brass rod 1s slid forward until it just touches the scale at the foot 
of the perpendicular from the mirror. The two sliders are then placed upon the 
original steel scale, from which the glass scale was calibrated, and moved until the 
fine lines on the end of the brass rod are seen sharply in focus. The distance between 
the engraved lines on the plate-glass bases, so determined, added to the distance 
between the engraved lines at 2 inch and 21 ft. 8 in. or 9 in., as the case may be, is 
MDCCCXCV.—-A. D 
