214 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



Fig. 219. 

 Fig. 218. 



rod movable, though perfectly air-tight, may be fixed ; on 

 the outer end of the rod there is a ring, serving as a handle 

 for depressing the rod, and within a hook for suspending 

 from. This, or a similar arrangement, is used with the 

 improved bell, flint and steel, &c., and parts of this may be 

 used for other experiments, the screws being made uniform 

 in their size. Price, complete, 



" plate, with slide rod, $3.00. 



Bell in Vacua. (Fig. 218, as above.) This is an appara- 

 tus to show that sound is not propagated in vacuo, and 

 thereby to prove that sound is but the air in vibration. It 

 consists of a small bell mounted within a glass receiver, and 

 supported from the top by a wire attached to a plug moving 

 air tight in a socket, and having a handle above the cap, by 

 which the bell may be rung ; when rung in a vacuum a 

 great diminution of the sound will take place, and were it 

 possible to produce a perfect vacuum, and insulate the bell 

 from all sonorous bodies, no sound would be heard. Three 

 sizes. Price $2.25, $2.50 and $3.00. 



Improved Bell in Vacuo. This form of the bell in vacuo 

 (Fig. 219, as above) has a spring-lever hammer, working 

 within the bell, and rung by depressing the rod having a 



