174 



AIR-PUMP, 



FIG. 108. 



FIG. 109. 



air exhausted. Remove the air, close the 



stop cock, place the lower end of the tube 



in water, open the stop -cock ; a beautiful 



fountain will be produced (Fig. 109). 

 (10.) The mercury shower apparatus 



consists of a cup through the bottom of 



which passes a plug of oak or other porous 

 wood. Place the cup upon 

 the Mnd-glass with a tum- 

 bler below ; pour some 

 mercury into the cup ; ex- 

 haust the air, and the at- 

 mospheric pressure will 

 force the mercury through 

 the pores of the wood. 



(11.) The weight-lifter 

 (Fig. 110) is an apparatus 



by means of which the pressure of the atmosphere may be made to 



lift quite a heavy weight. It consists of a stout glass cylinder, C, 



supported by a frame and tripod. Within the lower part of the 



cylinder is a closely fitting pis- 

 ton from which the weight is 



hung. A brass plate is ground 



to fit accurately upon the top 



of the cylinder. This plate is 



perforated and a flexible tube, 



B, connects the cylinder with 



an air-pump. When the air 



is exhausted from the cylin- 

 der, the atmospheric pressure 



on the lower surface of the 



piston raises the piston and 



supported weight the length 



of the cylinder. 



(12.) The Magdeburg hemi- 

 spheres are made of metal. 



They are hollow, and generally 



three or four inches in diam- 

 eter. Their edges are provided 



with projecting lips which fit 



one over the other. These 



edges fit one another air-tight ; 



the lips prevent them from FIG. no. 



