PNEUMATICS. 



arc found in liquids, are so inconsider- 

 able, that the quality of compressi- 

 bility in these substances is rather to be 

 looked upon as & philosophical fact, than 

 as a property to be taken into account 

 in mechanical investigations. Accord- 

 ingly, in all mechanical treatises, liquids 

 and aeriform bodies are considered to 

 be distinguished as we have already ex- 

 plained ; and when their properties are 

 expressed mathematically, the formulae 

 for the one are founded on the supposi- 

 tion of their being inelastic, and for the 

 other of their being elastic. 



The same property of incompressi- 

 bility which we have just explained in 

 water, is common to all that class of 

 fluids which are called liquids, such 

 as mercury, alcohol, &c. ; and the 

 property of elasticity explained in the 

 instance of air is common to all fluids 

 of the gaseous or vaporous form, such 

 as all the gases, steam raised by heat 

 from all species of liquids, &c. 



This manifest and important mecha- 

 nical distinction between the two classes 

 of fluids gives rise to a corresponding 

 division in that part of mechanical phi- 

 losophy which treats of their properties. 

 That which treats of the mechanical 

 properties of elastic fluids, and which 

 forms the subject of the present treatise, 

 is called Pneumatics, from the Greek 

 word friw/ua, (pneuma,) which signifies 

 breath or air. 



(4.) The various elastic fluids differ 

 one from another in many respects. One 

 of the most striking distinctions is, that 

 some are permanently elastic, and others 

 not. Those which are incapable by any 

 known means of being converted into a 

 liquid are called permanently elastic 

 fluids. Such, for example, is air. On 

 "the other hand, those which, by being 

 submitted to pressure or exposed to cold, 

 are reduced to liquids, are not perma- 

 nently elastic, and are generally called 

 vapours. Such, for example, is steam. 

 Besides this, there are many other dis- 

 tinctions between elastic fluids, arising 

 out of their chemical properties. It 

 will not, however, be necessary here to 

 inquire into these, since the mechanical 

 properties which we shall have to con- 

 sider in the present treatise are common 

 to all elastic fluids, whatever be their 

 differences as to permanent elasticity or 

 any other properties. The elastic fluid 

 with which we are most familiar is at- 

 mospheric air, and it possesses all the 

 mechanical properties which we shall 

 have to notice in any elastic fluid. For 



this and other reasons it will be conve- 

 nient to adopt it as the representative of 

 elastic fluids in general, and there will 

 be no difficulty in applying to them the 

 conclusions to which we may arrive. 



CHAPTER II. 



Air possesses the universal Properties 

 of Matter Impenetrability, Inertia^ 

 Mobility, and Weight. 



(5.) Air being apparently an invisible, 

 intangible substance in which we freely 

 move, it may at first be doubted whether 

 it be matter or not. It should be ob- 

 served, that the properties of substances, 

 even those with which we are most ha- 

 bitually conversant, do not always offer 

 themselves immediately to the observa- 

 tion of the senses, and that in noticing 

 them our senses must often be guided 

 by philosophical considerations. Not 

 that these philosophical considerations 

 add any thing to the certitude derived 

 from the senses, but rather that they 

 direct the senses to the proper objects of 

 attention. Let us then consider how we 

 should use the senses in deciding the 

 question, whether air be material or 

 not ? We know that there are certain 

 properties which any thing must have in 

 order to be material, and that having 

 these properties it is necessarily one of 

 that class of beings which we denote by 

 the term matter. Air, then, will be ma- 

 terial or not, according as it is found to 

 possess or not these requisite qualifica- 

 tions. The principal of these properties 

 are, impenetrability, inertia, mobility, 

 and weight. 



(6.) Impenetrability is that property 

 which a body occupies any space to 

 e exclusion of every other body, or so 

 that no other body can fill that space 

 until the other deserts it. 

 (7.) Air is impenetrable. 

 There are various experimental proofs 

 of this proposition. Let A B (fig 2.) be 

 a glass receiver, or cylindrical vessel, 

 containing water to the level B, and let 

 C D (fig. 3.) be a smaller vessel of the 

 same kind empty, and having 1 an aper- 

 ture in the bottom furnished with a stop- 

 cock at F. Let a cork or other light 

 body be placed floating on the surface of 

 the water at G, and the stop-cock F 

 being closed, let the vessel C D be in- 

 verted over the cork G, (as al.Jig 4.) 

 and let its mouth D be pressed into the 

 water to any convenient depth. It will 



