twang of a harpsichord wire, the agitation of a particle may be near the goth part of the extent of the undulation. This muft difturb the regularity of the motion, and caufe the agitations in the remote undulations to differ from thofe in the firft pulfe. In the explosion of a cannon, the breaking of an exhaufted bottle, and many inftances which may be given, the agitations are ftill greater. The commentators on Newton's Principia, Le Sueur and Jacquier, have fhown, and Euler more clearly, that when the original agitations are very violent, the particles of air will acquire a fubordinate vibration, compounded with the regular cycloidal vibration, and the progrefs of the pulfes will be fomewhat more rapid; but the intricacy of the calculus is so great, that they have not been able to determine with any tolerable precifion what the change of velocity will be. All this, however, is fully confirmed by experiment on founds. The found of a cannon at 10 or 20 miles distance does not in the least resemble its found when near, In this cafe it is a loud inftantaneous crack, to which we can affign no mufical pitch; at a diftance, it is a grave found, of which we can tell the note; and it begins foftly, fwells to its greatest loudnefs, and then dies away growling. The fame may be faid of a clap of thunder, which we know to be a loud fnap of ftill lefs duration. It is highly probable that the appreciable tone which thofe diftant founds afford is produced by the continuance of thefe fubordinate vibrations which are added together and fortified in the fucceffive pulfes, though not perceptible in the firft, in a way fomewhat refembling the refonance of a mufical chord. Newton's explanation gathers evidence therefore from this circumftance. And we muft further obferve, that all elaftic bodies tremble or vibrate almost precifely as a pendulum swinging in a cycloid, unlefs their vibrations are uncommonly violent; in which cafe they are quickly reduced to a moderate quantity by the refiftance of the air. The only very loud founds which we can produce in this way are from great bells; and in these the utmoft extent of the vibration is very fmall in comparison with the breadth of the pulfe. The velocity of these sounds has not been compared with that of cannon, or perhaps it would be found lefs, and an objection against Newton's determination removed, He gives 969 feet per fecond, Experiment 1142. We fhall now attend to the propagation of aerial pulfes as they really happen. Suppose a sphere A, fig. 61, pl. CCLXXIX. filled with condenfed air, and the veffel which contains it fuddenly annihilated. The air muft expand to its natural dimenfions, fuppofe BCD. But it cannot do this without preffing afide the furrounding air. We have feen that in any fingle row of particles this cannot be at once diffused to a diftance, but muft produce a condenfation in the air adjoining; which will be gradually propagated to a distance. Therefore this fphere BCD of the common denfity will form round it a fhell, bounded by EFG, of condenfed air. Suppofe that at this inftant the inner air BCD becomes folid, The fhell of condenfed air can expand only outwards. Let it expand till it is of the common density, occupying the fhell HIK. This expanfion, in like manner, muft produce fhell of condenfed air without it; at this infta let HIK become folid. The furrounding thell condenfed air can expand only outward, co denfing another fhell without it. It is plain that th muft go on continually, and the central agitatio will be gradually propagated to a diftance in a directions. But, in this procefs, it is not the fam numerical particles that go to a distance. Tho of the original sphere go no farther than BCI those of the next fhall go no further than HI &c. The expanfion outwards of any particle w be more moderate as the diffufion advances; f the whole motion of each shell cannot exceed th original quantity of motion; and the number particles in each fucceffive fhell increases as th furface, that is, as the fquare of the distance fro the centre; therefore the agitation of the partic will decrease in the fame ratio, or will be in th inverfe duplicate ratio of the diftance from th centre. Each fucceffive shell, therefore, contain the fame quantity of motion, and the fucceffi agitations of the particles of any row out from th centre will not be equal to the original agitation as happens in the folitary row. But this does no affect the velocity of the propagation, because a agitations are propagated equally faft. In the above we have fuppofed the air A to be come folid as foon as it acquired the common den fity; but this was only to facilitate the conceptio of the diffufion. It does not ftop at this bulk for while it was denfer, it had a tendency to ex pand. Therefore each particle has attained the diftance with an accelerated motion. It wil therefore, continue this motion like a pendulu that has paffed the perpendicular, till it is brough to reft by the air without it; and it is now rare than common air, and collapfes again by the greate elafticity of the air without it. This outward air therefore, in regaining its natural denfity, mu expand both ways. It expands towards the centre following the collapfing of the air within it; and expands outwards, condensing the air beyond it By expanding inwards, it will again condenfe the air within it, and this will again expand; a fimila motion happens in all the outward fhells; and thus there is propagated a fucceffion of condenfed and rarefied fhells of air, which gradually fwell to the greatest distance. We might demonstrate, that when the central air has for the fecond time acquired the natural denfity, it will be at reft, and be difturbed no more; and that this will happen to all the thells in fucceffion. But as this demonftration is too intricate, we shall only point out a fact perfectly analogous. When we drop a fmall pebble into water, we fee it produce a feries of circular waves, which go along the furface of smooth water to a great diftance, becoming more and more gentle as they recede from the centre; and the middle, where the agitation was first produced, remains perfectly smooth, and this fmoothness extends continually; that is, each wave when brought to a level remains at reft. Now, thefe waves are produced and propagated by the depreffion and elevation made at the centre. The elevation tends to diffuse itself; and the force with which each particle of water is actuated is a force acting directly directly up and down, and is proportional to the elevation or depreffion of the particle. This hydrofatical preffure operates precifely in the fame way as the condensation and rarefaction of the air; and the mathematical investigation of the propagation of the circular undulations on smooth water is fimilar in every ftep to that of the propagation of the spherical waves in ftill air. For this we appeal to Newton's Principia, or to Euler's Ola, where he gives a very beautiful inveftigation of the velocity of the aerial pulfes; and to fone memoirs of De la Grange in the collections of the academies of Berlin and Turin. These two laf authors have made the investigation as fimple as fees poffible, and have freed it from every objedi which can be ftated against the geometrical one of their great teacher Newton. The fhell of condensed air which comes againft the glafs has a great furface and a great agitation; the beft fecurity in this cafe is to throw up the fafh; this admits the condensed air into the room, which acts on the infide of the window, balancing part of the external impulfe. In every elementary treatife of natural philofophy, it is fhown, that when a wave on water meets any plane obftacle, it is reflected by it from a centre equally removed behind the obstacle; that waves radiating from one focus of an ellipfe are made to converge to the other focus, &c. All this may be affirmed of the aerial undulations; that when part of a wave gets through a hole in the obstacle, it becomes the centre of a new series of wave; that waves bend round the extremities of an obftacle; all this happens in the aerial undulations. And, laftly, that when the furface of water is thrown into regular undulations by one agitation, another agitation in another place will produce other regular waves, which will cross the former without difturbing them in the fmalleft degree. The fame thing happens in air; and experiments may be made on water which will illuftrate in the moft perfect manner many other affections of the aerial pulfes, which we fhould otherwife conceive very imperfectly. We would recommend to our curious readers to make fome of thefe experiments in a large veffel of milk. Take a long and narrow plate of lead, which, when fet on the bottom of the veffel will reach above the furface of the milk; bend this plate into a parabola, elliptical, or other curve. Make the undulations by dropping milk on the focus from a small pipe, which will cause the agitations to fucceed with rapidity, and then all that we have faid will be moft diftinctly feen, and the experiment will be very amufing and inftructive, especially to the mufical reader. This friking fimilarity between the aqueous waves and the aerial undulations, affords a very fenfible object to reprefent many affections of the latter, which it would be difficult to explain otherwife. We neither fee nor feel the aerial dalations; and they behoved, therefore, to be described very imperfectly without fuch aid. In the watery wave there is no permanent progreffive motion of the water from the centre. Throw a fmall bit of cork on the furface, and it will be oberved to popple up and down without the leaft action outwards. In like manner, the particles ofar are only agitated a very little outwards and da; which motion is communicated to the particles beyond them, while they themselves come to reft, unless agitated afresh; and this gration of the particles is inconceivably fmall. Even the explosion of a cannon at no great diftance will but gently agitate a feather, giving it a fingle impalfe outwards, and immediately after another wards or towards the cannon. When a harpfichord wire is forcibly twanged at a few feet ditance, the agitation of the air is next to infen- To conclude, we requeft all who read explanable. It is not, however, nothing; and it differs from tions of natural phenomena by means of vibrations that in a watery wave by being really outwards of athers, animal fpirits, nervous fluids, &c. to fix and inwards. In confequence of this, when the their attention on the nature of the agitation in condenfed fhell reaches an elaftic body, it impels one of thefe undulations. Let the inquifitive it lightly. If its elafticity be fuch as to make it reader confider whether this can produce the acquire the oppofite fhape at the inftant that the phenomenon, acting as any matter muft act, by next agitation and condenfed fhell of air touches it, impulfe or by preffure. If he fees that it can its agitation will be doubled, and a third agitation produce the phenomenon, he will be able to point wall increase it, and fo on, till it acquire the agita- out the very motion it will produce, both in tion competent to that of the fhell of air which quantity and direction, in the fame manner as Sir reaches it, and it is thrown into fenfible vibration, ISAAC NEWTON has pointed out all the irregula ad gives a found extremely faint indeed, because rities of the moon's motion produced by the The agitation which it acquires is that correfpond- difturbing force of the fun. In this, we are nga hell of air confiderably removed from the perfuaded, he will fail. Let him then try to point inal fring. Hence, a musical chord, pipe, or out fome palpable connection between the general bel, will aufe another to refound, whofe vibrations phenomena of elaftic undulations and the phenoare ifochronous with its own; or if the vibrations menon in question; this would fhow an accom of the one coincide with every 2d, 3d, or 4th, paniment to have at least some probability. It is &c. of the other; just as we can put a very heavy thus only we learn that the undulations of air pedulum into pull with the breath at every vibration, or at the mechanifm of the ear; but we fee that the neighbourhood of another drum will agitate it that certain modifications of the one are regularly 3d, 3d, or 4th, &c. A drum ftruck in the phenomena of found always accompany them, and confenfibly; for here the ftroke depreffes a very accompanied by certain modifications of the other. confiderable furface, and produces an agitation of In this attempt too he will fail; but let him a confiderable mafs of air; it will even agitate the remember that even if he fhould be able to cannon will even break a neighbouring window. duction of the phenomenon, the whole is ftill an furface of tagnant water. The explosion of a fhow the competency of this fluid to the pro bypothefis hypothefis, becaufe we do not know that fuch a fluid exists. Whoever will proceed in this prudent manner will foon fee the futility of moft explanations of this kind which have been given. Even confummate mathematicians, who should best understand the mechanifm of aerial undulations, fpeak of them as very imperfectly understood. But even the unlearned in this fcience can fee the incompatibility of the hypothefes, with many things which they are brought to explain. To take an inftance of the conveyance of fenfation along the nerves; an elaftic fluid is fuppofed to occupy them, and the undulations of this fluid are thought to be propagated along the nerves. Let us confider how the undulations would be conveyed along the furface of a canal, which was completely filled up with reeds and bulrushes, or let us make the experiment on fuch a canal: we may reft affured that the undulations in the one case will resemble thofe in the other; and we may fee that in the canal there will be no regular or fenfible propagation of the waves. We offer thefe obfervations to warn our readers againft this fashionable proneness to introduce invifible fluids and unknown vibrations into phyfical difcuffions. They have done immense mischief in fcience, and there is but one phenomenon that has ever received an explanation by their means. SECT. XI. Of the EFFECTS of the PRESSURE and ELASTICITY of the AIR. AMONG the various phenomena arifing from the preffure of the atmosphere, there is none more furprifing than the very great force and ftrong cohesion that take place between twofmooth furfaces, which accurately fit each other. (See COHESION, 1-5.) This is a fact familiarly known to the glafs-glinders, polishers of marble, &c. A large lense or speculum, ground on its tool till it becomes very smooth, requires more power than even a very strong man is endowed with, to feparate it directly from the tool. If the surface is only a fquare inch, it will require 15 pounds to separate them perpendicularly, though a very moderate force will make them flide along each other. But this cohefion is not observed unless the furfaces are wetted or smeared with oil or greafe; otherwise the air gets between them, and they feparate without trouble. That this cohefion is owing to the atmospheric preffure, is evident from the ease with which the plates may be feparated in an exhaufted receiver. To this caufe is also afcribed the very ftrong adhesion of fnails, periwinkles, limpets, and other univalve fhells, to the rocks. The animal forms the rim of its thell, fo as to fit the fhape of the rock to which it intends to cling. It then fills it fhell with water. In this condition we must act with a force equal to 15 lb. for every fquare inch of touching furface before we can detach it. This may be illuftrated by filling a drinking glafs to the brim with water; and having covered it with a piece of thin wet leather, turn it on a table, and then try to pull it ftraight up; it will require a confiderable force. But if we expofe a fail adhering to a ftone in the exhaufted receiver, we fall fee it drop off by its own weight. In the fame manner do the remora, the polypus, the lar prey, and many other animals, adbere with fuc firmnefs. Boys often amufe themfelves by pulli out large ftones from the pavement by means a circle of ftiff wetted leather faftened to a ftrin It is owing to the fame cause that the bival fhell fishes keep themselves fo firmly fhut. W think the mufcular force of an oyfter prodigiou because it requires fuch force to open it; but we grind off a bit of the convex fhell, fo as t make a hole in it, though without hurting ti fish in the smallest degree, it will open with gre ease, as it does alfo in vacuo In this way, the preffure of the air contribute much to the cohesion of bodies, where we do no fufpect its influence. The tenacity of our morta and cements would frequently be ineffectual with out this affiftance. It is owing to the preffure the atmosphere that a cafk will not run by th cock unless a hole be opened in some other par of the cafk. If the cafk is not quite full, fome 1 quor, indeed, will run out, but it will ftop as foc as the diminished elasticity of the air above th liquor is in equilibro (together with the liquor with the atmospheric preffure. In like manner a tea-pot muft have a small hole'in its lid to en fure its pouring out the tea. If indeed the hol in the cask is of large dimenfions, it will run with out any other hole, because air will get in at the up per fide of the hole, while the liquor runs out by th lower part of it. On the fame principle depend the performance of an inftrument ufed by fpirit dealers and excifemen, for taking out a fample o their fpirits. It confifts of a long tinplate tube AB. (fig.62. Pl. 281, open at top at A, and ending in a fmall hole at B. The end B is dipped into the fpirits, which rises into the tube; then the thumb is clapt on the mouth A, and the whole is lifted out of the cafk. The spirit remains in it till the thumb be taken off, when it is allowed to run into a glafs for examination. It is chiefly owing to the preffure of the air that frofts immediately occafion a fcantinefs of water in our fountains and wells. This is erroneously accounted for, by fuppofing that the water freezes in the bowels of the earth. But the moft intense frost of a Siberian winter would not freeze the ground two feet deep; yet a very moderate froft will confolidate the whole furface of a country, and make it impervious to the air; efpecially if the froft has been preceded by rain, which has foaked the furface. When this happens, the water which was filtering through the ground is all arrefted, and kept fufpended in its capillary tubes by the preffure of the air, in the very fame manner as the fpirits are kept fufpended in the inftrument just described by the thumb's thutting the hole A. A thaw melts the fuperficial ice, and allows the water to run in the fame manner as the fpirits run when the thumb is removed. Amolpheric or common air is neceflary for fupporting the lives of moft animals. If a fmall ani mal, fuch as a moufe or bird, be put under the receiver of an air-pump, and the air be exhausted, the animal will quickly be thrown into convul fions and fall down dead; if the air be immedi ately re-admitted, the animal will fometimes revive, especially if the rarefaction has been-briky made made, and has not been very great. We do not know that any breathing animal can bear the air to be reduced to of its ordinary density, nor even; nor have we any good evidence that an animal will ever recover if the rarefaction is pushed very, although continued for a very fhort time. the mere prefence of the air is by no means ficient for preferving the life of the animal; for it is found, that an animal fhut up in a veffel of air cannot live in it for any length of time. If a man be fhut up in a box, containing a wine hoghead of air, he cannot live in it much above n hour, and long before this he will find his breathing very unfatisfactory and uneafy. A galJon of air will fupport him a minute. A box EF Pl. 181.) may be made, having a pipe AB alerted into its top, and fitted with a very light ve at B, opening upwards. This pipe fends of lateral branch a D dC, which enters the box the bottom, and is alfo fitted with a light valve Copening upwards. If a perfon breathe through the pipe, keeping his noftris fhut, it is vident that the air which he expires will not enter the box by the hole B, nor return through the pipe CDd; and by this contrivance he will grabaly employ the whole air of the box. With this apparatus, experiments can be made withont any risk or inconveniency, and the quantity of air ellary for a given time of eafy breathing, may be accurately afcertained. know that the lime has combined with the fixed air. The celebrated Dr STEPHEN HALES made many experiments, to clear the air from the noxious vapour which he fuppofed to be emitted from the lungs. He made ufe of the apparatus above defcribed, and he put several divifions of thin woollen ftuff into the box, and moiftened them with various liquids. He found nothing fo efficacious as a folution of potash. If the folation is not already faturated with a fixed air, it will take it up as fast as it is produced, and thus will purify the air: à folution of cauftic alkali, therefore, will have this effect till it is rendered quite mild. Thefe experiments have been repeated, and våried in many circumftances, to afcertain whether this fixed air was really emitted by the lungs, or whether the infpired air was in part changed into fixed air by its combination with fome other fubftance. This is a question which the doctrines of pneumatics enable us to anfwer. If the fixed air be emitted in fubftance from the lungs, it does not appear how a renewal of the air into which it is emitted is neceffary; for this does not hinder the fubfequent emiffion, and the bulk of the air would be increafed by breathing in it, viz. by the bulk of all the fixed air emitted; but, on the contrary, it is a little diminished. We must therefore adopt the other opinion; and the difcoveries in modern chemistry enable us to give a pretty accurate acHow the air of our atmofphere produces this count of the whole procefs. Fixed air is a com, is a queftion not eafily decided. But it is pound, of which one ingredient is oxygen, which either the preffure nor the elafticity of the air, conftitutes about of the whole atmospheric fluid. which is immediately concerned in preferving When this is combined with charcoal, the refult is animal life. We can live and breathe with free- fixed air or carbonic acid. The change therefore dom on the tops of the higheft mountains. The which breathing makes on the air, is the folution alley of Quito in Peru, and the country round of this matter by vital air; and the ufe of air in Condor in Abyffinia, are fo far elevated above the breathing is the carrying off this noxious principle Surface of the ocean, that the preffure and the in folution. When therefore the air is already fo elafticity of the air are one third lefs than in the far faturated as not to diffolve this fubftance as falt low countries; yet thefe are populous and heal- as it is fecreted, or must be fecreted in the lungs, thy places. When an animal has breathed in the animal fuffers the pain of fuffocation, or is any quantity of air for a certain time without re- otherwife mortally affected. Suffocation is not the newal, it will not only be fuffocated, but another only confequence; for we can remain for a numanimal put into this air will die immediately; yet ber of feconds without breathing, and then we beTether the preffure nor elafticity of the air is re- gin to feel the true pain of fuffocation; but thofe markably diminifhed. Reftoring the former pref. who have been inftantaneously ftruck down by an fure and elafticity has not the fmalleft tendency inspiration of fixed air, and afterwards recovered to prevent the death of the animal; for an ani- to life, complained of no fuch pain, and feemed ma will live no longer under a receiver that to have fuffered chiefly by a nervous affection. has its mouth inverted on water, than in one But thefe are queftions out of our prefent line of 1 upon the pump-plate covered with leather. inquiry. See ANATOMY, BLOOD, 7, 8. GHBfrew when the receiver is fet on water, the pref- MISTRY, Index; FIXED AIR; MEDICINE, Index tue of the atmosphere acts completely on the in- the GASES, under PHARMACY, Appendix; and elafticity. manner the preffure and elasticity of the air, com cluded air, and preferves it in the fame ftate of PHYSIOLOGY. Our business is to explain in what hort, the air which has already ferved to bined with the structure and mechanifm of the boand alimentary properties completely changed, and removal of the matter difcharged from the maplain the animal functions, has its chemical dy, operate in producing this neceffary fecretion and is no longer fit for this purpose. So much of lungs in the act of breathing. any mafs of air as has already been thus employed, The fecretion is made from the mafs of blood is changed into what is called fixed air by Dr Black, during its paffage through the lungs. The blood Of carbonic acid by modern cherhifts. Any perfon delivered into the lungs is of a dark blackish còmay be convinced of this by breathing or blowing lour, and it is there' changed into a florid red. In through a pipe immersed in lime water. Every the lungs it is exposed to the action of the air in a expiration will produce white clouds in the wa- prodigiously extended furface: for the lungs con tated in the form of pure chalk. In this cafe we bladders, communicating with each other and with the the windpipe. These are filled with air in every infpiration. Thefe veffels are every where in contact with minute-blood veffels. The blood does not in toto come into immediate contact with the air; and it would feem that it is only the thin ferous part of it which is acted on by the air at the mouths of the veffels or pores, where it ftands by capillary attraction. Dr Prieftly found, that venous blood inclosed in thin bladders and other membranes, was rendered florid by keeping the bladders in contact with pure vital air. Breath is moist or damp, and must acquire this moisture in the lungs. It is immaterial whether this fecretion of water or lymph be furnished by mere exudation through fimple pores, or by a vascular and organic fecretion; in either cafe, fome ingredient of the blood comes in contact with air in the lungs, and there unites with it. This is farther confirm ed, by observing, that all breathing animals are warmer than the furrounding medium, and that, by every process in which carbonic acid gas is formed from oxygen, heat is produced. Hence this folution in air of fomething from the blood has been affigned by many as the fource of animal heat. We touch on thefe things, in a tranfitory way, only to prove that, for the fupport of animal life, there must be a very extenfive application of AIR to the blood, and that this is made in the lungs. This is done in two ways; by the action of the muscles of the ribs, and by the action of the diaphraghm and other muscles of the abdomen. The thorax or cheft is a great cavity completely filled by the lungs. The fides of this cavity are formed by the ribs. These are crooked or arched, and each is moveable round its two ends, one of them being inferted into the vertebræ of the back, and the other into the fternum or breaft-bone. The rib turns in a manner refembling the handle of a drawer. See fig. 64, pl. 281. The curves ace, bkf, clg, &c. reprefent the ribs moveable round the extremities. Each fucceeding rib is more bent than the one above it, and this curvature is both in the vertical and horizontal direction. Each is fo broad as to project a little over its inferior like the tiles of a roof. If we take the lower one by its middle, and draw it out a little, moving it round the line np, it will bring out the next amb along with it. As the diftance of the middle point o from the axis of motion np is greater than the diftance of m from the axis d b, and as o will therefore describe a portion of a larger circle than m does, the rib nop will flide up a little under the rib d m h, or the rib d m h will overlap n op a little more than before; the diftance om will therefore be diminished. The fame muft happen to all the fuperior ribs; but the change of diftance will be lefs and lefs upwards. Now, inftead of this great breadth of the ribs over-lapping each other, fuppofe each inferior rib connected with the one above it by threads or fibres fufceptible of contraction at the will of man. The articulations e, a, of the firft or upper rib with the spine and fternum are fo broad and firm, that this rib can have little or no motion round the line a e; this rib, therefore, is as a fixture for the ends of all the contracting fibres: therefore, whenever the fibres which connect the rib with the first rib contract, the ad must rife a little, and alfo go outward, and w carry the lower ribs along with it; the 3d rib w rife ftill farther by the contraction of the mufc) which connect it with the fecond, and so on: a thus the whole ribs are raised and thrown outwar (and a little forward, because the articulation each with the spine is confiderably higher than th with the fternum), and the capacity of the thor is enlarged by the contraction of its mufcular c vering. The direction of the muscular fibres very oblique to the direction of the circular m tion which it produces; from which circumftan it follows, that a very minute contraction of th muscles produces all the motion which is neceff ry. This indeed is not great; the whole motio of the loweft ribs is less than an inch in the mo violent inspiration, and the whole contraction the mufcles of the 12 ribs does not exceed the 8 part of an inch, even fuppofing the intercoft mufcles at right angles to the ribs; and being of lique, the contraction is ftill lefs. (See BORELL SABBATIER, MONRO, &c.) It would feem, the the intensity of the contractive power of a mufcu lar fibre is eafily obtained, but that the fpac through which it can be exerted, is very limited for in moft cafes the mufcles are placed in fitua tions of great mechanical disadvantage in this re fpect, to procure other conveniences. Nor is this the whole effect of the contractio of the intercoftal mufcles: As the compound ac tion of the two sets of muscles, which crofs eac other from rib to rib, like X, is, nearly at righ angles to the rib, but is oblique to its plane, tends to push the ribs closer on their articulations and thus to prefs out the two pillars on which they are articulated. Thus, fuppofing a ƒ (fig 65. Pl. 281.) to reprefent the section of one o the vertebræ of the spine, and cd a section of the fternum, and a bc, fed, two oppofite ribs, with a lax thread be connecting them. If this thread be pulled upwards by the middle g till it is tight it will tend to pull the points b and e nearer to each other, and to press the vertebra dƒ, and the fternum cd outwards. The fpine being the chief pillar of the body, may be confidered as immoveable in the prefent inftance. The fternum is fufficiently fufceptible of motion for the present purpofe. It remains almoft fixed a-top at its articulation with the firft rib, but it gradually yields below; and thus the capacity of the thorax is enlarged in this direction alfo. The whole enlarge ment of the diameters of the thorax during inspi ration is very fmall, not exceeding the goth part of an inch in ordinary cafes. This is eafily cal culated. Its quiefcent capacity is about two cu bic feet, and we never draw in more than 15 inches. Two fpheres, one of which holds a cubic feet, and the other 2 feet and 15 inches, will not differ in diameter above the soth part of an inch. 2 The other method of enlarging the capacity of the thorax is very different. It is feparated from the abdomen by a ftrong mufcular partition called the diaphragm, which is attached to firm parte all around. In its relaxed state, it is confiderably convex upwards, that is, towards the thorax, rifing up into its cavity like the bottom of a quart battle. Many of its fibres tend from its middle to |