PNEUMATICS. 



THIS branch of science treats of the nature and properties of the atmosphere, 

 and of their effects upon solid and fluid bodies. The atmosphere is a thin 

 gaseous substance, which envelopes the earth on every side to the height of about 

 forty-five miles, its density decreasing with its height. Fig. 1, represents the 

 atmosphere, which is divided by lines into thirty spaces, each of which contains 

 the same quantity of air, the lower layers being so much compressed by the 

 weight of those above them, that the lower half of the atmosphere lies within 

 about three and a half miles of the earth's surface, while the upper half is so 

 expanded as to occupy upwards of forty miles. The upper thirtieth part alone 

 occupies more space than all the remaining thirty-nine parts. 



Mechanical Properties of Air. The most essential point in which air 

 differs from other fluids is by its elasticity ; that is, its power of increasing or 

 diminishing its bulk, according as it is less or more compressed. Air possesses 

 the universal properties of matter. Its impenetrability may thus be shown : 

 Fig. 2, is a vessel partly filled with water, upon the surface of which floats a 

 small cork ; fig. 3 is a smaller cylindrical glass vessel, with a stop cock, which is 

 closed. If this vessel be inverted over the cork, as at fig. 4, and its mouth 

 pressed into the water, it will be fonud that the water will not enter the inverted 

 glass, except to a very limited height, owing to the air in the glass excluding the 

 water ; but if the stop cock be opened, the air will escape, and the water rise to 

 the same level within as it is without the glass. 



That air is inert and moveable, we have many and familiar proofs, as the 

 resistance it offers to a body passing through it, and the force exerted by the 

 wind. It also possesses weight, one hundred cubic inches weighing about thirty- 

 one grains. 



Laws of Air- First, the pressure of the air is equal in all directions ; second, 

 its degree of pressure depends on the vertical height, and is in proportion to its 

 density, and to the weight of the fluid displaced. 



That air presses in all directions may be proved by filling a bladder with that 

 fluid, and then pressing upon it; the pressure will be freely communicated 

 through the mass, and the confined air will rush out with equal force at whatever 

 part a hole is made in the surface. 



The pressure depending on vertical height or depth of air, is an important 

 property of the atmosphere, and on it depends the explanation of numerous 

 phenomena. 



Air being a substance possessing gravity, necessarily presses downwards in the 

 direction of the centre of the earth ; and, therefore, the degree of pressure on, 

 any given point will be equal to the weight of the column of air above that 

 point, and proportional to its density. The atmosphere is of the greatest 

 vertical height at the level of the sea, and here its pressure is about fifteen 

 pounds on every square inch of surface. The pressure being exerted upwards 

 sideways, obliquely, and in every other direction, as well as downwards. 



Some illustrations may be given of the pressure of the air. Figs. 5 and 6 

 represent two hollow hemispheres of brass ; these being placed in contact and the 

 air withdrawn from the interior, the external air will exert a pressure of 15 Ibs. 

 upon every square inch of their surface, so that if two persons pull the handles 

 in opposite directions they will be unable to separate the hemispheres. The 

 common leather sucker (fig. 7) with which boys raise stones, acts from the 

 pressure of the atmosphere. It is the pressure of the atmosphere which causes 

 liquids to rise in pumps and syphons. 



The Barometer- This instrument consists of a column of mercury, sup- 

 ported in a tube by the pressure of the atmosphere, and therefore indicating that 

 degree of pressure (fig. 8). It is formed by a glass tube about 34 inches long, 

 closed at one end and open at the other. The tube being filled with mercury, the 

 open end is stopped with the finger to prevent any running out, and the tube 

 being inverted, the open end is placed in a small cup of mercury, and the finger 

 being withdrawn, the mercury in the tube now subsides three or four inches, 

 above the top of which in the tube is a perfect vacuum. The tube being then 

 fixed to a graduated frame, we have a barometer. The mercury will stand in the 

 tube at the height of from 28 to 30 inches, according to the state of the air ; and 

 the reason of this is, that the pressure of the whole atmosphere will not raise 



