AIR PUMPS AND APPARATUS. 215 



cap on the top, by means of a sliding rod attached to the 

 receiver. The bell is mounted on a neat metal foot, and 

 placed on a round piece of wood, or leather, when used ; 

 to prevent, as far as possible, the transmission of the sound. 



Price, $3.00. 



Float Wheel for showing the resistance Fig. 220. 

 of the air, Fig. 220 consists of two light \ 

 metal floats, or vanes, fixed to a centre and 

 turning freely on a pin fixed in a handle. 

 These vanes may be so placed on the pin 

 that their whole surface may strike the air ; 

 or, on the contrary way, that only the 

 edge of the vanes may strike the air. To 

 use, place the vanes on the pin so that the whole surface 

 strikes the air, and give it motion with the finger, and it will 

 soon stop, owing to the resistance of the air. Place it now 

 in the position in which the edge only strikes the air, and 

 give it motion, and it will be continued for several minutes. 



Price, $0.75. 



Torricellian Experiment. (Fig. 221, next page.) A stout 

 barometer tube of thirty-one inches in length, hermetically 

 sealed at the upper end ; the lower end immersed in a cup 

 of mercury ; the whole covered by a tall bell glass receiver. 

 On exhausting the air in the receiver, the air in the tube is 

 expanded, and escapes through the mercury ; on returning 

 the air to the receiver, the mercury is supported in the tube 

 to a height proportioned to the degree of exhaustion obtained 

 therein. Price, with receiver, $5.00. 



Torricellian Experiment with Sliding Tube. (Fig. 222, 

 next page.) This drawing represents a more perfect arrange- 

 ment for the Torricellian experiment. The tube is supported 

 in the neck of a swelled bell glass receiver, having a brass 

 cap with packing-box and screw, containing collars of leather 

 through which the tube slides readily. To use, the tube is 

 exhausted with the end out of the mercury, and then im- 

 mersed by sliding down the tube through the collar of 

 leathers ; on returning the air to the receiver the mercury 

 rises in the tube, showing the exact degree of exhaustion 

 obtained. Price, $3.50 and $4.00. 



