24 



MECHANICS, 



other properties of air, its fluidity and 

 its elasticity. By the former it trans- 

 mits the pressure arising from the 

 weight of the incumbent atmosphere 

 equally in every direction, so that it is 

 not only an horizontal surface which 

 sustains the pressure of lolbs. per inch, 

 but surfaces in all possible directions 

 and positions suffer the same pressure. 

 Also, by reason of air being an elastic 

 fluid, it expands itself, so as to fill every 

 open space not actually occupied by 

 other bodies, whether solid or fluid. 

 Consequently in the case we have sup- 

 posed, air must occupy the space in the 

 cylinder below the piston as well as 

 above it, and if so, the fluidity of the 

 air will transmit the pressure arising 

 from the weight of the atmosphere to 

 the lower surface of the piston with un- 

 diminished force, and thus we shall 

 have the piston pressed upwards and 

 downwards with equal forces, and con- 

 sequently no mechanical advantage 

 will be obtained. 



(54.) It appears, therefore, that before 

 the weight of the atmosphere, whether 

 acting immediately downwards, or trans- 

 mitted laterally, obliquely, or upwards, 

 by means of its fluidity, can be used as 

 a mechanical agent, it is indispensably 

 necessary that the air be removed from 

 the other side of the body on which this 

 weight or pressure is designed to act. 

 Recurring to the example of a piston in 

 a cylinder, it is necessary to remove the 

 air from one side of the piston before 

 its weight or pressure can take effect 

 upon the other side. Now if this re- 

 moval, as is often the case, be effected 

 by mechanical means, it must, on the 

 slightest consideration, be quite appa- 

 rent that it will require exactly as much 

 force to remove the air from one side 

 of the piston, as will be subsequently 

 gained by the pressure of the atmos- 

 phere on the other tide. Suppose, for 

 example, that from two feet in length 

 of the cylinder below the piston, the air 

 which it originally contained be with- 

 drawn by mechanical force. To effect 

 this will require a force of at least 15lbs, 

 for every square inch in the section of 

 the cylinder, acting through the space 

 of two feet, and after it has been effected 

 the piston will be forced into the vacuum 

 with exactly the same force. 



It appears, therefore, that in order 

 to render the atmospheric pressure an 

 available mechanical agent, a vacuum, 

 or a partial vacuum, must always be 

 produced; and further, that if this 



vacuum, or rarefaction, be produced by 

 mechanical means, no power will be 

 gained, since it will always require as 

 much force to accomplish this, as will 

 be exerted by the atmospheric pressure 

 when it has been accomplished. In 

 the use of mechanism, however, the 

 gaining of power is not always the end 

 to be attained. It is frequently a matter 

 of great convenience, and, in a certain 

 sense, of great mechanical advantage, 

 to be able, by a power which acts in 

 some particular manner, to obtain ano- 

 ther equal power, whose mode of action 

 may be different, and better suited to 

 the purpose to which mechanical agency 

 is to be applied. This is, in fact, the 

 case in every instance in which the 

 atmospheric pressure is obtained by 

 mechanical rarefaction, and in every 

 such case the atmospheric pressure 

 should not be looked upon as the prime 

 mover, but rather as an intermediate 

 agent deriving its entire efficacy from 

 that power, whatever it may be, which 

 is used to produce the rarefaction. A 

 most obvious instance of this may be 

 observed in the common suction-pump, 

 described in our Treatise on Pneumatics, 

 Art. 40. This machine is introduced 

 into that treatise, not because it owes 

 its original mechanical efficacy to the 

 pneumatical principle of atmospheric 

 pressure, but because this principle is 

 involved in the detail of its operation. 

 In this machine, the first mover is the 

 power, whatever it be, which works the 

 piston. This power, at the commence- 

 ment of the operation, produces a rare- 

 faction in the space between the piston 

 and the surface of the water in the well. 

 The weight of the atmosphere acting 

 upon the external surface of the water 

 in the well forces into the pump-barrel 

 just so much water as the power applied 

 to the pump-rod would have been ca- 

 pable of lifting, were it immediately 

 applied to that purpose. This appears 

 very evident from the investigation 

 contained in Art. 42. PNEUMATICS. 



What we have observed of the suc- 

 tion-pump may be applied in general to 

 all cases where the atmospheric pres- 

 sure receives its efficacy from mecha- 

 nical rarefaction. Strictly speaking, we 

 cannot consider the atmospheric pres- 

 sure as a first mover at all ; the first 

 mover is the cause, whatever it be, 

 whether mechanical or otherwise, which 

 produces the rarefaction. 



(55.) By that quality called inertia, 

 air, when in motion, exerts a force upon 



