210 CAPTAIN SABINE ON THE REDUCTION TO A VACUUM 



the point of suspension was immoveable, that the small motion of the support 

 of the pendulum, occasioned by the elasticity of the iron bars, did not preju- 

 dice the comparative result of the vibration in air of different density: so 

 that in all cases where a relative result only is required, the apparatus is effec- 

 tive, without the means resorted to at Greenwich to make the suspension im- 

 moveable. 



Through the liberality of the Managers of the Royal Institution, the use of 

 the air-pump belonging to the Institution was obtained for these experiments : 

 and also an apparatus for the formation and supply of hydrogen gas, for pur- 

 poses which will be described in their due succession. The experiments will 

 be related in the order in which they were made, as being perhaps the most 

 simple and perspicuous mode. 



June 28, 1828. — On this day Mr. Newman having brought all the parts of 

 the apparatus intended for the vacuum experiments to Portland Place, it was 

 established in front of Mr. Browne's clock by Molvneux. The invariable 

 pendulum No. 12. was placed on the agate planes numbered also 12, which were 

 screwed fast to the iron plate supported by the four iron bars, and were care- 

 fully levelled. The thermometer, graduated by Mr. Daniell and myself, used 

 in my former pendulum experiments, was suspended withinside the glasses, so 

 that the ball was midway between the axis and the lower part of the weight 

 of the pendulum ; a mercurial gauge, commencing to act when the pressure 

 was reduced to 10 inches, was also suspended. The pendulum was 1 foot 6 

 inches in front of the pendulum of the clock • and the telescope for observing 

 coincidences was stationed in an adjoining room, 18 feet 6 inches from the 

 detached pendulum, and 20 feet from the pendulum of the clock. A detached 

 diaphragm, having a vertical opening, the sides of which viewed from the tele- 

 scope were tangents to the disk, was placed between the vacuum apparatus 

 and the disk ; so that when the glasses were on, the disk and diaphragm were 

 both seen from the telescope through the back and front of the lower glass 

 cylinder. A graduated arc was placed with the diaphragm : the distance from 

 the axis to the part of the pendulum crossed by the arc as seen from the 

 telescope was 49.5 inches : the arc was divided into degrees, and the degrees 

 were subdivided into spaces of 1 0' each : the length of a degree was 0.73 inch : 

 consequently the arc read off required to be multiplied by .845 to produce 



