PNEUMATICS. 



great force which is necessary to destroy 

 or modify the motion of air. 



If the atmosphere be quiescent and 

 there be no wind, there is a resistance 

 opposed by it to the passage of any body 

 through it. This resistance is produced 

 by the force exerted by the body so 

 passing through it in displacing the air 

 in its progress. When the motion is 

 slow, and the surface of the body which 

 faces the direction of the motion not 

 very great in proportion to the whole 

 body, this resistance is perhaps not sen- 

 sibly felt. But if the motion be accele- 

 rated or the surface much enlarged, it is 

 instantly perceived. In walking at a 

 moderate pace on a calm day we do not 

 easily feel the resistance of the air, but 

 if we increase our speed and run, we 

 find' the same effect as if a wind were 

 blowing in our face ; this effect increases 

 in proportion to the speed, and is very 

 obvious w r hen riding or driving ra- 



If a large fan or an open umbrella be 

 moved slowly against the calm air, the 

 resistance will be instantly felt, and a 

 considerable exertion will be necessary 

 to sustain the motion. 



(10.) Air has weight. 



If our object be merely to establish 

 the fact that air is heavy, the most 

 direct method is to weigh it by the 

 usual means, a balance. But if it be 

 required to ascertain with great nicety 

 its degree of weight or specific gravity, 

 other less direct but more accurate 

 means rhust be resorted to. In the 

 present instance we shall be content 

 with establishing the fact that air has 

 weight. 



There are some very obvious effects 

 which plainly indicate this. It is shown 

 in our treatise on Hydrostatics, that when 

 a lighter body is placed in a fluid it as- 

 cends in it, and that if it so ascend it 

 must be lighter than the fluid in which 

 it moves. Now there can be no doubt 

 that a balloon has weight, and yet it 

 ascends in the atmosphere. The atmos- 

 phere must then, bulk for bulk, be hea- 

 vier than the balloon. Besides this, the 

 clouds which we see floating in the at- 

 mosphere are generally composed of 

 water, as is proved by their frequently 

 falling in rain. They have therefore 

 weight, but yet must be lighter than the 

 atmosphere in which they are sus- 

 pended. 



If a piston move in a cylinder so as 

 to be air-tight and be provided with a 

 valve which opens upwards, upon 



pressing the piston to the bottom of the 

 cylinder, the air contained in the cylin- 

 der will be forced through the valve in 

 the piston. Let us then suppose the 

 piston in close contact with the bottom 

 and sides of the cylinder, all air having 

 been excluded : upon attempting to 

 draw the piston up, it will be found that 

 very considerable force will be neces- 

 sary ; and that when sufficient effort has 

 been used, and the piston has been 

 brought to the top of the cylinder, if it 

 b3 disengaged from the agent which 

 drew it up, it will descend with great 

 force and strike the bottom. This 

 effect plainly indicates the weight of the 

 air pressing on the upper surface of the 

 piston. This is what is vulgarly called 

 suction ; as if there were some force 

 within the cylinder which drew the pis- 

 ton to the bottom. But within the cy- 

 linder is nothing but empty space, and 

 it is plainly unreasonable to ascribe to 

 empty space any mechanical influence. 

 ; That it is the weight of the incumbent 

 atmosphere pressing on the upper sur- 

 face of the piston which forces it to the 

 bottom of the cylinder, is still further 

 proved by the fact, that if the upper 

 surface of the piston be increased, the 

 force which presses it down will be also 

 increased, and what is more, will be in- 

 creased in precisely the same proportion 

 as the surface of the piston. In fact, 

 it is found that when all air or other 

 elastic fluid has been expelled from be- 

 neath the piston, there will be a pressure 

 amounting to about fifteen pounds on 

 every square inch of the upper surface 

 of the piston ; from which we may infer 

 that a column of air, having a square 

 inch for its base, and which extends 

 from the surface of the earth to the top 

 of the atmosphere, weighs about fifteen 

 pounds*. The atmospheric engine is a 

 machine whose efficacy depends on the 

 principle which we have been just ex- 

 plaining. In this machine the weight 

 of the atmosphere is used as a first 

 mover in pressing a piston to the bottom 

 of a cylinder}'. 



* Its weight varies within narrow limits, as will 

 appear hereafter. Fifteen pounds may be considered 

 an average in round numbers. 



f In calculating the atmospheric pressure on any 

 surface, it is necessary to determine the number of 

 square inches in the surface. As it often happens 

 that the surface thus to be measured is circular, it 

 may be useful to have a short and easy rule, by which 

 the number of square inches in a circle of a given 

 diameter may be found. The following rule will be 

 found to serve this purpose: 



Let half the diameter of the circle expressed in 

 inches be found, and its square taken, and let this 

 square be multiplied by the number 3.1415, and the 



