JM 



A III. 



AIR-ENG I 



let 



great, any more than ft wrt sponge would tie deranged by pressing 

 any depth in water." (' Mechanical Philosophy,' vol. iii. p. 541.) 



The temperature of air, a* already noticed, influence* it* elastic 

 force. We have every reason to conclude, tout the principal properties 

 ni thin and all other gases are a consequence of the preaenoe of heat 

 though we do not know what the latter in. It is probable that air 

 w..uld become tint liquid, and then Holid, if it could lie mail.- -utli 

 i ii-ntly cold. Like all other substances, air give* out heat when it is 

 compressed, that i, raises the temperature of mirruiinding bodies, am 

 rift rend. This it atrikingly illustrated by the fact that tinder can be 

 net on fire when the air contained in a brass cy)in<l<-r i- suddenly am' 

 violently compressed by a piston. 



From careful experiment* it appears, that air and all other gases, as 

 well a* vapourd, and also all mixture* of gases and vapour*, obtain an 



of elutic force for every increase of temperature, and expand 

 therefore, if expansion be possible, in the vessel which contains them: 

 The quantity of this expaiwion, when the tem|>erature passes from the 

 freezing to the boiling point of water (that is, from 32 to 212 ol 

 Kahrenhrit't). from to 80 of Reaumur's, and from 0" to 100 of 

 the Centigrade, thermometer*), in 366 parts out of a thousand of the 

 bulk which it bad at the freezing point. That in, in the apparatus 

 indicated in the preceding part of this article, form a graduated scale 

 along B A, and suppoee that 11 K contains a thousand parts, and tint . tin- 

 upper air being removed, as much mercury is jwured in above E r as 

 will cause the membrane EF to stand at K, when tin- temperature o) 

 the air is at the freezing point of water. Then, if the air be gradually 

 heated from the freezing to the boiling point of water, either more 

 mercury muat be poured in, or the membrane with the sujierinoumlient 

 mercury will rise through 366 divisions more of the scale, and K will 

 stand at 1366. And thi* dilatation is uniform : that is, whatever 

 expansion arise* from an increase of 12 of temperature, half as much 

 arises from an increase of 6', twice as much from one of 24, an 

 This remarkable law, which holds, with perhaps a slight variation, at 

 very high and very low temperatures, was discovered nearly at the 

 same time by Dalton in England and Oay-Lussac in France. Now, in 

 Fahrenheit's thermometer there are 212 82, or 180 between the 

 boiling and freezing points of water ; 80 in Reaumur's ; and 100 in 

 the Centigrade. Consequently, the whole increase of bulk, or jjjjjj, will 

 P ve rtSfto tjSbt md ,,JJI.,, for the variations of bulk corresponding to a 

 rise of one degree of temperature on each of the three thermometers ; 

 that is, about j$ t , jj ; , and ,,, respectively. But in applying these 

 rules, it must be recollected that, in taking Fahrenheit's thermometer, 

 for example, the expansion is jj t of the bulk irliir/i it had at the freezing 

 point. Suppose, for instance, we have a bulk of air which occupies 

 1000 cubic inches at the temperature of 62 Fahrenheit, and we wish 

 to know how much it would occupy under the same pressure at 82 of 

 the same. The first temperature is 30 above the freezing point. Now, 

 suppose a certain volume of air to -consist of 491 parts at the freezing 

 point; then it is clear that, as air expands ? J t of its bulk for each 

 degree, this volume at 62 will have become 491 + 30 = 521 part*. 

 Similarly, at 82 it will be 491 + 50 = 541 parts. Hence, as 521 (the 

 volume at 62) : 1000 (also at 62), so is 541 (the volume at 82) : 

 1088J, which is the bulk required at 82. 



On the properties of air with regard to other bodies, we may notice 

 that probably there is a slight adhesion of air to many, if not to all, 

 tin-faces. A small needle may be made to swim on water, and in this 

 state the water evidently retires from around it, leaving it, as it were, 

 impended over a hollow in the fluid. This is attributed to the 

 adhesion of a coat of air, which, with the iron, makes the whole 

 specifically lighter than the water. Recent experiments on the pen- 

 dulum, the most delicate of all philosophical instruments, have led 

 ome to suspect, that in addition to the resistance of the air, a slight 

 coating of this substance travels with the pendulum, and thereby 

 causes an irregular addition to its weight [PENDULUM.] 



The air is a permanent gas, incapable of being reduced to the liquid 

 state by cold or pressure. It i* also, like most gases, perfectly colour- 

 lex, especially when we look through small quantities of it ; although, 

 if we notice the effect produced by large masses of it, we may consider 

 it to be a coloured gn*. Thus, the blue colour of the sky is probably 

 merely the colour of the air seen through a length of about 4!> mil.-. 

 Hence, it has been observed by those who have ascended about 5 miles 

 from the earth's surface, when they have left much more than half 

 the atmosphere behind them, that the sky appears of a dark inky hue. 

 owing to the very small reflection .m.l ili-|M-i>jon of the UK"', while the 

 blue colour no longer appear* abort, but brloK them. Similarly, the 

 blue colour of distant hills is owing to the same cause. 



In this article we have considered only the chemical and mechanical 

 properties of air. The constitution of the whole mass will come under 

 the article ATMOSPIIFHK. To complete the subject, refer to OXTOKC, 

 RnrtRATiox, Comnmox, Vr-mtATiox, ACOUSTICS, AERODYNAMIC*. 

 and also to the ' Element* of f 'hemistry,' by Professor Miller, of King's 

 College, London, and to the ' Cours de Chimie,' by Regnault. 



AIR, in music, signifies ' Melody ;' the terms are synonymous, it 

 bring understood that by both words is meant a succession of single 

 sounds in measured time. The word Air was used in this sense nearly 

 three centuries ago ; hut it is not now known why such an application 

 of a familiar won) was first adopted. 



Rousseau says that the name of air is given to all melodies, to dis- 



tinguish them from recitative. M. Suard, in th* ' Encyclopedic Me"- 

 thodique,' offers the following definition : a piece of music, composed 

 of a certain numlx-r of melodious phrases, united in a rcgul.i: 

 metrical form, and terminating in the key in which it Ix-gaii. 

 h:u- followed M. Suard ; so lias I'ietro l.ichtcnthal : but, wit 

 ing to his definition, we consider tin- cuminon .mil .-ii, {| .l, . tin- 

 best, namely, that succession of single sounds, regulated liy the- l.m 

 nf unified rhythm, which con-titute* what, in limm-lv 

 called a tune. ( KHVTIIM.] 



Air, or melody, is, allo\v,-dly. the must imjxirtaiit of tin- ei.n-tituent-. 

 of musir. A ei>iii|N>sitiiiii may !< replete with learned and ingeniou- 

 harmony, may al>oiinil in fugue, in imitation, ami all thi contrivances 

 of science, but without good melody will never appeal to the heart, 

 and seldom afford any gratification to the car. Haydn carried thi.i 

 opinion so far as to say, " Let your air be good, and y.mr i-mii). 

 whatever it may be, will possess beauty, and mly please." 



Air is in music what design and outline are in the PI mint- 



ing: harmony i* the filling up, and the colouring. 



The Greeks had many kinds of airs, which tln-y c-.dl, .: 

 songs ; and we learn from the work of IMiilodcmus on 

 from the ruins of Herculaneum, that every trade and iK-ciipatiun had 

 it* No/ui, or appropriate airs, which were played or sung to the- work- 

 men while they laboured. 



The various kinds of airs, instrumental as well a* vocal, will In- f,,nnd 

 under their different heads. [AI.I.KMANIIK ; KAKOARUI.I.E; & 

 music composed for the theatre, and which is constantly introduced 

 into the concert-room, are the following varieties .f air, designated by 

 Italian denominations, viz. 



The Aria tli l.'aratlrrr ( characteristic air'), which is distinc 

 by force and energy of expression, and l.y dramatic i-tl'.-.-i. The Ariit 

 Parlante ('speaking air'), which is rather declaimed than sung, and is 

 best suited to the buffo or comic performer. The.ln'n iti (',i H t<i>,!l, 

 (' singing air 'I, a tender, pathetic; air, calling forth the expression and 

 taste of the singer. The Aria '/i llmr,ira ('dashing air'). an ;iir in 

 which the- performer displays his powers of cxcvntion. rather 



to astonish than please. 



AIR-CUSHIONS. The mechanical application of common air, in 

 respect of its pressure or elasticity, has been greatly extended within 

 the lost few years. Provided a m:i.-s ..f air can IK' confined within a 

 given receptacle, and that receptacle be of an elastic or yielding 

 character, the air assumes many of the qualities of a soft still! 

 padding, when its quantity is small compared with the size 

 envelope; but when the quantity is as great as can be intrixliiciil 

 without bursting the envelo]>e, the air becomes nearly equivalei:' 

 solid body. So long as the means were wanting for conveniently 

 making air-tight cloth vessels,' this principle was slenderly applied ; but 

 the use of caoutchouc or India-rublicr, as a glutinous varnish, lias 

 developed many ingenious contrivances for thi When a bag 



has been made of such material, rendered also air-tight by somewhat 

 similar means at the seams, air may be passed into it as 

 for more solid materials. In practice there are some very neat, 

 arrangements adopted in effecting this. Temporary air-seats or cushions 

 arc made by forming a bag of air-tight cloth, perfe. 1 1 \ enclosed at every 

 part except one corner, where is inserted a small tub* and stop 

 capable of effecting or preventing communication from the interior 

 to the exterior. The cock being opened, and the tulx- applied 

 mouth, air is blown into the cushion, until it expands to the . 

 degree of fulness; the cock is then closed, and the air remain 

 prisoned. When not in use, such a cushion can have the air expressed 

 from it, and may then be folded up into a small space. 



It is obvious that seats, cushions, pillows, and beds of various kinds, 

 having a similar object in view in relation to softness, t 

 elasticity, may be made by similar means. When the quai 

 included in an envelope is greatly increased, it may be made the t 

 of producing actual pressure in a more equable way than by any --h.l 

 bodies. Thus, an air-tight bandage was invented a few years ago. tli 

 object of which was to form a wrapper to a human limb under surgical 

 treatment. There is a bandage with straps fitted for being tied round 

 ;he limb; and when so tied, air is breathed into an air-tigh 

 :o the bandage, through a small tube provided with n pifx 1 . By 

 ncrcasing or lessening the quantity of air impelled, the pressure of the 

 mndage on the limb may be made so small as to be scarcely perceptible, 

 or may on the other hand be made even painfully close ; but in - 

 case it will be equable in every port. 



A patent has been obtained, by Mr. Walton, for certain modifications 

 n the mode of preparing air-bags for beds and similar purposes. 

 (..How balls of India-rubber, filled with air, ore enclosed within 

 external coverings so as to form a sheet of balls ; and many such 

 may IK' piled one on another to form a bed or mattram of any required 

 thickness. Such an arrangement would, however, be very complicated 

 and expensive. 



FnrtliKT illustrations bearing on this subject will be met with under 

 PE-BoAT and WATERPROOF COMPOSITION-. 

 AIR KNCIXKS. Many attempts have been made to pr. 

 engines which should have the power of steam-engines without tli 

 of steam. The compression or the rarefaction of air, brought about in 

 some one of many different ways, is the agent relied upon fi 

 T moving force. In 1840 Mr. Stirling patented such a machii; 



