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from within will swell the skin and cause it to be sucked into the glass. This 

 experiment may be performed by the mouth on the flesh of the hand or arm. 

 If the lips be applied to the flesh, and the breath drawn in so as to produce a 

 partial vacuum in the mouth, the skin will be drawn or sucked into the mouth. 

 This effect is owing, not to any force resident in the lips or the mouth drawing 

 the skin in, but to the fact that the usual external pressure is removed, and 

 that the piessure from within is suffered to prevail. 



All cases of that class of effects which are commonly expressed by the 

 word suction are accounted for in the same manner. 



If a flat piece of moist leather be put in close contact with a heavy body, as 

 a stone, it will be found to adhere to it with considerable force, and if a cord 

 of sufficient length be attached to the centre of the leather, the stone may be 

 raised by the cord. This effect arises from the exclusion of the air between 

 the leather and the stone. The weight of the atmosphere presses their sur- 

 faces together with a force amounting to fifteen pounds on every square inch 

 of those surfaces in contact. If the weight of the stone be less than the num- 

 ber of pounds which would be expressed by multiplying the number of square 

 inches on the surfaces of contact by fifteen, then the stone may be raised by 

 the leather ; but if the stone exceed this weight, it will not suffer itself to be el- 

 evated by these means. 



'.^he power of flies and other insects to walk on ceilings and surfaces pre- 

 sented downward, or upon smooth panes of glass in an upright position, is said 

 to depend on the formation of their feet. This is such that they act in the 

 manner above described respecting the leather attached to a stone ; the feet, in 

 fact, act as suckers, excluding the air between them and the surface with which 

 they are in contact, and the atmospheric pressure keeps the animal in its po- 

 sition. In the same manner the hydrostatic pressure attaches fishes to rocks. 



The pressure and elasticity of the air are both exercised in the act of 

 breathing. When we draw in the breath we first make an enlarged space in 

 the chest. The pressure of the external atmosphere then forces air into this 

 space so as to fill it. By a muscular action the lungs are next compressed so 

 as to give this air a greater elasticity than the pressure of the external atmo- 

 sphere. By the excess of this elasticity it is propelled, and escapes by the 

 mouth and nose. It is obvious, therefore, that the air enters the lungs not by 

 any direct act of these upon it, but by the weight of the atmosphere forcing it 

 into an empty space, and that it is expired by the action of the lungs in com- 

 pressing it. 



The action of common bellows is precisely similar, except that the aperture 

 at which the air is drawn in is different from that at which it is expelled. In the 

 lower board of the bellows is a hole covered by a valve, consisting of a flat 

 piece of stiff leather, moveable on a hinge, and which lies on the hole, but is 

 capable of being raised by a slight pressure. When the upper board of the 

 bellows is raised, the internal cavity is suddenly enlarged, arid the air contained 

 in it is considerably rarefied. The pressure of the atmosphere forces in air at 

 the nozzle, but this being too small to allow its admission with sufficient ease 

 and speed, the valve covering the hole is acted upon by the atmosphere and 

 raised, aud air rushes in through the large aperture under it. When the space 

 between the boards is filled with air in its common state, the upper board is 

 depressed, and the air confined in the bellows is suddenly condensed. The 

 valve covering the hole is thus kept firmly closed, and the air has no escape 

 except through the nozzle, from which it issues with a force proportioned to the 

 pressure exerted on the upper board. A bellows, such as that in common do- 

 mestic use, thus simply constructed, has an intermitting action and blows by 

 fits, its action being suspended while the upper board is being raised. T " 



In 



