BLADDER GLASS MAGDEBURG HEMISPHERES. 



with the bottle described in our Tract on Air,* already quoted. 

 The air included in a room is not directly under the pressure of a 

 column of atmosphere extending indefinitely upwards ; but it is 

 compressed at all the openings by which it enters the room^by the 

 external air, and this pressure, which proceeds from the incum- 

 bent weight of such an atmospheric column outside the room or 

 building, being freely transmitted inwards, the air in the room is 

 affected by it in exactly the same manner, and to exactly the 

 same degree, as if it were placed immediately under the incumbent 

 weight of a column extending to the top of the atmosphere. 



5. There is another familiar experiment which illustrates in a 

 striking and instructive manner the atmospheric pressure, and 

 which is known as that of the Magdeburg hemi- 

 spheres. The apparatus known by this title is 

 represented in fig. 1. It consists of two hollow 

 brass hemispheres with evenly ground edges, 

 which admit of being brought into air-tight con- 

 tact when smeared with lard. The apparatus 

 when secured upon the plate of an air-pump may 

 be exhausted, so that the space within the hemi- 

 spheres may be rendered a partial vacuum. The 

 external air will thus press the two hemispheres 

 together with a force proportional to the difference 

 between the pressure of the external air and the 

 pressure of the rarefied air within. When a 

 sufficient exhaustion has been produced, the stop- 

 cock attached to the lower hemisphere is closed, 

 the apparatus is unscrewed from the pump-plate, 

 and a handle screwed upon the lower hemisphere. It will be 

 found that two of the strongest men will be unable to tear the 

 hemispheres asunder, provided they are of .a moderate magnitude, 

 owing to the amount of the pressure with which they are held 

 together. If, for example, the pressure of the rarefied air 

 within is equivalent to a column of two inches of mercury, 

 while the external air has a pressure represented by 30 inches of 

 mercury, there will be a force amounting to 14 Ib. per square 

 inch on the section of the hemispheres. 



If the hemisphere have 4 inches diameter, the area of their 

 section will be 1 2| square inches, and consequently the force with 

 which they will be pressed together will be 

 12 X 14 = 175lb. 



This apparatus derives its name from the place where the 

 inventor of the air-pump, Otto Guericke, first exhibited the 



* Vol. ii., p. 4. 



H2 



