208 CAPTAIN SABINE ON THE REDUCTION TO A VACUUM 



frame by which the suspension is fixed securely to the wall of the apartment. 

 The pedestal is of cast iron 2 inches thick, being a cylinder of one foot in 

 height and a foot interior diameter, open at the top, and closed at the bottom 

 by an horizontal plate 3 feet long by 16 inches broad, resting on four screws, 

 by which it can be raised, depressed, and levelled. A metallic pipe commu- 

 nicating with an air-pump is fitted to a hole perforated at half the height of 

 the cylinder. The metallic pipe is furnished with a stop-cock, by which the 

 communication between the pump and the interior of the cylinder can be closed 

 at pleasure. The three next pieces in succession above the pedestal are glass 

 cylinders slightly conical, having their rims ground into surfaces fitting one to 

 another. The suspension piece, which is the next above the three glasses, is a 

 metal plate, having holes to receive the screws of the bed containing the agate 

 planes, and to admit the pendulum to its place : it is surrounded by a circular 

 metal ring, the outside of which forms a part of the exterior of the apparatus, 

 and the upper and lower surfaces are ground, so as to form close joints with 

 the glass cylinder beneath, and with a bell glass, which is the 6th piece, com- 

 pleting the upper part of the apparatus. The ring surrounding the suspension 

 plate is perforated to admit a screw, which passes through a stuffing-box, and 

 acts on an inclined plane beneath the Ys, serving to raise the pendulum on 

 the Ys, and to lower it on the planes. The pedestal cylinder is also perforated, 

 to admit a wire with a cross-piece at the extremity, for the purpose of stopping, 

 or of giving fresh impulse to, the pendulum. This wire also passes through a 

 stufiing-box. 



To set up the apparatus, the pedestal is placed as nearly as can be judged in 

 the situation it will occupy when the suspension piece is secured to the wall. 

 A graduated arc is then fixed, by a wooden frame fitted to the interior of the 

 cylinder, so that the arc may be seen from the coincidence telescope about 2 

 inches above the iron cylinder. The three glasses are then placed in succes- 

 sion resting on one another, and the lower one resting on the ground surface 

 of the rim of the iron cylinder, the joints being made secure by pomatum. 

 The foot screws of the pedestal are then adjusted, until the upper glass is 

 brought exactly into its proper position, in regard to the iron frame by which 

 the apparatus is ultimately to be secured to the wall. The suspension piece is 

 then placed on the upper glass, on which it rests with its entire weight, ensu- 



