460 CAPTAIN SABINE ON THE LENGTH OF THE SECONDS PENDULUM 
tain Kater, and found to measure 39.44085 inches of that scale at the tempe- 
rature of 62°. 
The experiments were made, unless where it is otherwise noticed, in the 
vacuum apparatus, established in the S.W. angle of the pendulum room in the 
Royal Observatory, being the place assigned for that apparatus by the Astro- 
nomer Royal. 
The thermometer, by which the temperature of the pendulum was observed, 
is the same which I have used and described on former occasions ; particularly 
in the Phil. Trans, for 1830, where its comparison is given with a standard 
thermometer of M. Bessel’s, at the same temperatures at which it was used in 
these experiments. In a paper on the construction and use of the vacuum 
apparatus, in the Phil. Trans, for 1829, 1 have shown that when the pressure of 
the air is withdrawn from the exterior of the bulb of this thermometer, an 
index correction of + 0°.75 is required to make its indications in a highly 
rarified medium correspond with its measure of the same temperatures when 
under the pressure of the atmosphere. This index correction is consequently 
applied whenever the air is withdrawn from the apparatus. The thermometer 
was inclosed with the pendulum within the glasses, with its bulb suspended 
midway between the knife edges. When the air is withdrawn from the appa- 
ratus, and the shutters of the apartment are kept closed (except when light is 
required for the observation of coincidences), a great uniformity as well as 
steadiness of temperature is maintained within the glasses. 
Whenever the indications of the barometer were required, a reference was 
made to the standard barometer of the observatory, which is stationed in an 
adjoining room on the same level. 
The scale by which the arcs of vibration were observed was graduated in 
degrees, each degree being the 0.833th of an inch. In making the observation, 
the division of the scale coinciding with one side of the bar was noticed at each 
extremity of the vibration, and the same repeated with the other side of the 
bar ; a mean was then taken of the two included spaces, and half the mean 
registered as the arc on either side the vertical. The scale being placed in 
both positions of the pendulum 45.5 inches below the point of suspension, the 
registered arcs multiplied by 1.05 produce the true arcs of vibration ; by em- 
ploying these in the usual formula for that purpose, what is usually considered 
