172 



HISTORY OF SCIENCE. 



-cussion arose as to the possible existence of a vacuum. It was there- 

 fore natural that some should busy themselves to devise a contrivance 

 by which a vacuum could be produced. OTTO VON GUERICKE, bur- 

 gomaster of Magdeburg, it was who solved the problem, and gave us 

 another scientific instrument in the Air-Pump. Like every other dis- 

 covery of importance, this one was arrived at only by repeated trials 

 and failures. The first idea was to fill a barrel with water, and having 

 completely closed the vessel, except that a tube connected the interior 

 with a pump, to remove the water and thus leave an empty space. 

 When this was done with a barrel strong enough to resist the external 

 pressure, Guericke found that the air passed through the pores of the 

 wood. He then bethought himself of enclosing the barrel in a larger 

 one and filling the space between the two with water. The air still 

 found its way into the interior of the vessel. He then had made a 

 large hollow metallic globe, and without using water this time, he 

 connected this with a pump. The atmospheric pressure was perceived 

 in the effort required to work the pump : the resistance, trifling at first, 

 increased at every stroke, until the united strength of two men could 

 hardly accomplish the task. But when the exhaustion had been car- 

 ried to a considerable extent, the vessel suddenly collapsed with a loud 

 report, and was crushed as completely as if it had been thrown down 

 from the top of a lofty tower. A second apparatus more carefully 

 constructed gave, however, the result sought for. When finally a strong 

 glass globe had been substituted for the metallic sphere, in order that 

 the effects of a vacuum might be observed, the invention of the air- 

 pump as a scientific instrument was complete. The air-pump as ar- 

 ranged by Guericke consisted of a glass globe connected with the 

 barrel of the pump placed vertically beneath the globe. The pump 

 was worked by a lever, and the whole was supported 

 on a strong frame, as represented in Fig. 73. 



The principle on which this apparatus acted will 

 easily be understood by help of the diagram Fig. 73, 

 where A represents the globe to be exhausted, and 

 B c the cylinder, in which the piston D moves up 

 and down. If the piston be drawn down from B to 

 c, the air which before only occupied the space A B 

 expands so as to fill the cylinder as well as the globe. 

 If we suppose the cylinder to have twice the capa- 

 city of the globe, the air will thus fill three times its 

 original space. When the piston is at c, let us imagine 

 that all communication between the cylinder and 

 the globe is cut off at a. It is plain that the globe 

 would then contain only one-third of the original air, 

 and if the piston is then brought to its former posi- 

 tion by pushing it to the top of the barrel, after having opened at B a 

 communication with the external air, in order to allow the air impri- 



