216 CAPTAIN SABINE ON THE REDUCTION TO A VACUUM 



at 72", is 5.88 vibrations. The difference, or 3.845 vibrations per diem, is the 

 amount by which the experimental reduction to a vacuum exceeds the reduction 

 which it has been customary to compute. 



From the imperfect state of the apparatus in this first experiment, doubts 

 might have been entertained of the correctness of the result on two accounts : 

 it might have been supposed, 1st, that the abstraction of the air being kept up 

 continually, during the vibration in the rarefied medium, to counteract the 

 leakage, currents might have been occasioned influencing the time of vibration: 

 or, 2nd, the iron bars supporting the pendulum not having sufficient spread 

 at the bottom to counteract the lateral force arising from the vibration, and 

 the point of suspension itself partaking of it in consequence, it might have 

 been supposed that the time of vibration was unequally affected thereby in the 

 air and in the rarefied medium. By experiments made with the same pendu- 

 lum on the 8th and 9th of July, an account of which is already before the 

 Society, (Phil. Trans. 1 829, Art. IX.) in which experiments the pendulum was 

 suspended from Captain Kater's original mahogany suppoi't in the same room, 

 the vibrations on an immoveable support, all other circumstances being the 

 same, were found to exceed those on the plate of the vacuum apparatus, by about 

 18 vibrations a day; due, doubtless, to the motion of the plate during the 

 vibration, arising from the elasticity of the iron bars and their insufficient 

 spread. To give more firmness to the suspension in the vacuum apparatus in 

 a second experiment, inch boards of well seasoned oak were inserted verti- 

 cally, having their lower ends resting on the interior of the iron cylinder 

 which supports the glasses, and the plate was screwed down firmly on their 

 upper ends by screws working into tlie iron bars : the suspension plate was 

 thus directly and firmly connected with the foot cylinder by means of the 

 boards independently of the bars ; the boards being hollowed out in the proper 

 places to admit the observation of coincidences. 



To detect where the leakage took place, the interior of the apparatus was 

 filled with water as high as the lower glass cylinder, and a communication 

 being established between the exhausting pipe and the upper part of the in- 

 terior, the air was withdrawn ; when bubbles of air were seen to rise rapidly 

 from the interior surface of the iron foot cylinder, particularly from those parts 

 of it which were opposite the flanches on the outside, where the metal was 



