AIR PUMPS AND APPARATUS. 



213 



Balance Beam and Cork Ball F '^ 215 - 



(Fig. 215.) This consists of a bal- 

 ance, to one end of which is sus- 

 pended a piece of lead, in equilibrio 

 with a piece of cork, or other light 

 material, at the other end of the 

 beam. Place the beam and stand 

 under a receiver, and having ex- 

 hausted the air, the cork will pre- 

 ponderate ; for, as its bulk is greater 

 than that of the piece of lead, it 

 must be more sustained by the air ; 

 re-admit the air, and the equilibrium 

 will be restored. Price, $3.50. 



Sliding Rod Receiver (Fig. 216.) A 

 receiver for the air pump of any convenient 

 size or shape, having a brass cap cemented 

 to the neck, and a rod passing air tight 

 through a collar of leathers secured in a 

 cavity, and covered with a screw ; the rod 

 has a ring or handle at the top, and a hook 

 on the end within the receiver. One of the 

 many experiments for which this receiver is 

 used is the following : suspend a small re- 

 ceiver from the hook of this, and exhaust it 

 of air ; now let down the small receiver in 

 contact with the plate. While the outer one 

 is fixed, the inner one may be easily moved, 

 but on letting in the air, the inner is fixed 

 and the outer one loose ; observe in this ex- 

 periment the small receiver must not cover < 

 the hole in the centre of the receiver plate, 

 but be placed at the side. 



Price, with plain receiver, $3.50 and $4.00. 

 " with swell " $4.00 and $4.50. 



The Bell Glass Receiver with Sliding Rod. This piece of 

 apparatus (Fig. 217, next page) consists of a swelled bell 

 glass receiver, usually about 10 inches in diameter, having 

 an opening in the top of 2i to 3 inches in diameter, accu- 

 rately ground to fit a brass plate of a little larger size, hav- 

 ing a screw in the centre, into which a packing-box, with 



