280 MAGDEBURG HEMISPHERES 



fit exactly one upon the other; both are provided with 

 handles, and one with a lube which may be screwed 

 to the projecting tube of the plate, and can be closed 

 by a stop-cock. The edges being smeared with lard, 

 the hemispheres are placed upon one another, screwed 

 to the plate of the air-pump, and exhausted, so that the 

 space between them is rendered a partial vacuum, 

 The external air will then press the two hemispheres 

 together with a force equal to the difference between 

 the pressure of the . external air and the pressure of 

 the rarefied air within ; the amount of the compressing 

 force may be tested by seizing the handles and 

 attempting to separate the hemispheres, after the stop- 

 cock has been closed and the apparatus unscrewed 

 from the pump. If the diameter of the interior be 5 cm , 

 the force required to separate the hemispheres will be 

 about 20 kgr ; with a diameter of IQ cm it is 80 kgr , and if 

 the diameter is 20 cm it will be as much as 300 kgr , as can 

 be easily calculated by finding the area of the circular 

 section through the middle of the interior, and multi- 

 plying the number of square centimetres in this area 

 by the atmospheric pressure upon 1 square centimetre 

 (about l kgr ). The pressure of the enclosed air is so 

 small, when the rarefaction is as great as the pump 

 will permit, that it may be neglected in the calculation. 



The edges of the hemispheres should not be flat all round, but 

 one of them should be provided with a projecting fillet, which pre- 

 vents the possibility of lateral sliding. The force applied should 

 not effect an actual separation of the hemispheres, which might be 

 injured by striking against something, and thus spoiled ; they may 

 easily be separated, after opening the stop-cock, by turning one upon 

 the other with a moderate force, required to overcome the adhesion. 

 which makes them still hold pretty firmly together after the air is 

 let in. 



