212 
CAPTAIN SABINE ON THE REDUCTION TO A VACUUM 
meters of the Royal Society and of the Royal Observatory, made by means of 
an intermediate portable barometer, an index correction of -]- 0.066 was found 
to be required, to make Mr. Browne’s barometer agree with the standards 
when corrected for capillary action. This correction is accordingly applied. 
In these and in all the subsequent experiments, both in London and at Green- 
wich, the registered arcs were obtained in the following manner : — The points 
of the graduated arc were noted, opposite to which the same side of the tail- 
piece of the pendulum stopped when at each extremity of its vibration ; this 
gave the whole arc passed through by the pendulum without reference to a 
zero point : the half of this is the semi-arc of vibration. The same process was 
then gone through with the other side of the tail-piece of the pendulum ; and 
a mean of the two semi-arcs is the arc registered. The true arc is the registered 
arc multiplied by .845. 
The preliminary experiment in air being concluded, the pendulum was 
stopped by the wire passing through the stuffing-box, and again set in motion 
by the same at a true arc exceeding 1° 47', being the arc with which the pre- 
ceding experiment in air had commenced. The air was then withdrawn until 
the pressure was reduced to 7 inches. The thermometer, which had stood at 
71°.4 before the process of withdrawing the air commenced, was observed to 
fall gradually, until it was reduced to 70°-7, when the pressure Avas 7 inches. 
The interval between successive coincidences being about six minutes, and the 
leak admitting sufficient air to cause the gauge to rise an inch in six minutes, 
whilst by working the pump gently, tAVo inches could with ease be gained in 
the same time, the pump was worked, during the first 12 coincidences, only 
in the alternate intervals. From the 12th to the 39th coincidence, the gauge 
Avas kept always as near 7 inches as could be appreciated, by a very gentle 
and continued exercise of the pump. 
