PNEUMATICS. 



a column of mercury higher than about 30 inches j that is, a column of air equa* 

 to the height of the atmosphere, from the level of the sea, is of the same weight 

 as a column of mercury 30 inches high, the one thus balancing the other. The 

 figures at the sides of fig. 1, show the height of a column of mercury at different 

 elevations : the barometer thus becomes an important means of determining the 

 altitude of mountains. 



The wheel barometer is represented at fig. 9. The tube is closed at the top, 

 and bent upwards at its lower extremity, which is open, and the mercury buoys 

 ip a small float, F, to which a thread is attached, passing over a pulley and ter- 

 minating in the little ball, w. The friction of the thread on the pulley turns an 

 index which points to the words on the dial plate. 



When the mercury falls in the barometer, an indication is given of diminished 

 pressure ; and as this causes the air to expand and become sensibly cooled, 

 moisture is likely to be precipitated in the form of rain. 



The Ail Pump- Air may be artificially withdrawn from a containing ves- 

 sel by means of an apparatus called the air pump, represented at figs. 10 and 11 

 (the latter showing the pistons and valves). A is the receiver, resting in close 

 contact with the pump plate B, near the centre of which is the open end of the 

 tube c c, communicating with the exhausting barrels D D ; these are fitted with 

 pistons having valves opening upwards, so as to allow the air beneath to pass out 

 but preventing its return. At the bottom of the barrels are two other- valves 

 also opening upwards, admitting the air from the tube into the barrels when the 

 pistons are raised, and on their descent preventing its return ; the air thus con- 

 fined in the barrel, by the descent of the piston, becomes compressed, and forc- 

 ing open the valve in the piston, escapes into the open air. The pistons are con- 

 nected by a rack and pinion movement with a handle, and are raised alternately, 

 thus producing a vacuum beneath the receiver. By means of the air pump many 

 interesting experiments in pneumatics may be performed. When the air is 

 thoroughly exhausted, light and heavy bodies fall with equal swiftness: most 

 animals die immediately ; vegetation stops ; combustion ceases ; gunpowder will 

 not explode ; heat is slightly transmitted ; a bell sounds faintly ; magnets are 

 powerless ; glowworms give no light ; and watery and other fluids turn to 

 vapour. We thus see the important uses of the air in supporting life, vegetation, 

 and combustion ; in forming a medium for conveying to us the sound of each 

 others voices ; besides contributing in numberless ways to our comfort and 

 enjoyment. 



Air Condenser* Fig' 12 represents a section of the condensing syringe, 

 having an opening at A to admit the air, and a valve at B opening downwards. 

 The air being forced by the piston through the valve B, is prevented from return- 

 ing by the form of the latter. Fig. 13 represents a vessel partly filled with 

 water. By means of the condensing syringe, a large quantity of air may be 

 forced through the tube into the space A A ; the stop cock being then closed and 

 the syringe detached, and the stop cock being again opened, the pressure of the 

 air upon the surface of the water will force it up in the form of a jet d'eau. The 

 elastic force of compressed air is very great, and is sometimes employed for the 

 projection of balls from the air gun (fig. 14). 



Air Balloon- The air balloon (fig. 15) is a light silken bag of large dimen. 

 ions, filled with a gas, which bulk for bulk, is lighter than air, so that, when in- 

 flated, the machine becomes lighter than the air which it displaces, and this differ- 

 ence is so considerable that it is enabled to carry up with it several persons in a 

 car attached. As it ascends, the air becoming less dense, the difference between 

 its weight and that of the air displaced is gradually diminished, until it attains 

 such a height that the air it displaces is so rare, as to be only equal in weight to 

 the balloon; this, therefore, becomes the limit of its ascent. In order to descend 

 the bulk of the balloon is diminished, by the gas being allowed to escape by open- 

 ing a valve ; thus, the weight of the balloon is made to exceed that of an equal 

 bulk of air and it accordingly falls. 



The Diving Bell- Fig. 16. This machine is formed of iron, and is usually 

 capacious enough to hold three or four persons. It is constructed on the impene- 

 trability of air, before described. Air is pumped in from above by means of a 

 forcing air pump,or condenser ; the water is thereby prevented from rising in 

 the machine, and the persons within are enabled to breathe freely. A represents 

 the pipe conveying the fresh air from force pumps, and B the pipe conveying the 

 ritiated air from the bell. 



