THE AIR-PUMP. 



air. This apparatus is well known, and consists of two hollow copper hemi- 

 spheres which fit very closely. By means of the air-pump which he invented 

 in 1650, Otto von Guerike exhausted the air from 

 the closed hemispheres. So long as air remained in 

 them, there was no great difficulty in separating them ; 

 but when it had been finally exhausted, the pressure 

 of the surrounding atmosphere was so great that the 

 hollow spheres could not be dragged asunder even 

 by horses harnessed to rings which had been inserted 

 in the globes. 



The Air-Pump is a very useful machine, and we 

 will now briefly explain its action. The inventor was, 

 as remarked above, Otto von Guerike, of Magdeburg. 

 The pump consists of a cylinder and piston and 

 rod, with two valves opening upwards one valve F i g . 4 6.-Ma g debur g Hemi- 

 being in the bottom of the cylinder, the other in the spheres, 



piston. This pump is attached by a tube to a plate with a hole in it, 

 one extremity of the tube being fixed in the centre of the plate, and 

 the other at the valve at the bottom of the cylinder. A glass shade, called 

 the receiver, is placed on the top of the plate, and of course this shade will be 

 full of air (fig. 47). 



When the receiver is in position- we begin to work the pump. We 

 have said there are two valves. So when the piston is 

 drawn up, the cylinder would be quite empty did not the 

 valve at the bottom, opening upwards, admit some air 

 from the glass shade through the tube to enter the cylin- 

 der. Now the lower part of the cylinder is full of air 

 drawn from the glass shade. When we press the piston 

 down again, we press against the air in it, which, being 

 -compressed, tries to escape. It cannot go back, because 



Fig. 47. The air-pump 



the valve at the bottom of the cylinder won't open, so it escapes by the 

 valve in the piston, and goes away. Thus a certain amount of air is got 

 rid of at each stroke of the piston. Two cylinders and pistons can be 

 used, and so by means of cog-wheels, etc., the air may be rapidly exhausted 

 from the receiver. Many experiments are made with the assistance of the air- 

 pump and receiver, though the air is never entirely exhausted from the glass. 



The " Sprengel " air-pump is used to create an almost perfect vacuum, 

 by putting a vessel to be exhausted in connection with the vacuum at the 

 top of a tube of mercury thirty inches high. Some air will bubble out, and 

 the mercury will fall. By filling up again and repeating the process, the air 

 vessel will in time be completely exhausted. This is done by Mr. Sprengel's 

 pump, and a practically perfect vacuum is obtained,like the Torricellian vacuum. 



The " Torricellian vacuum " is the empty space above the column of 

 mercury in the barometer which we will proceed to describe. Air has a 

 certain weight or pressure which is sufficient to raise a column of mercury thirty 



