fure. Effects of the doctrines of pneumatics enable us to anfwer. If be furnished by mere exudation through fimple pores, It is well afcertained, that the fecretion is made from the mafs of blood during its paffage through the lungs. The blood delivered into the lungs is of a dark blackish colour, and it is there changed into a florid red. In the lungs it is expofed to the action of the air in a prodigiously extended furface: for the lungs confift of an inconceivable number of fmall veffels or bladders, communicating with each other and with the windpipe. These are filled with air in every infpiration. Thefe veffels are everywhere 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 Priestley found, that venous blood inclofed in thin bladders and other membranes was rendered florid by keeping the bladders in contact with abundance of pure vital air. We know alfo, that breath is moist or damp, and must have acquired this moisture in the lungs. It is immaterial whether this fecretion of water or lymph (as the anatomists call it) Matter a Plate The queftion before us in this place is, How is this brought about by the weight and elasticity of the air? This is done in two ways; by the action of the muscles of the ribs, and by the action of the diaphragm and other mufcles 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 breast-bone. The rib turns in a manner resembling the hand', drawer. The infpection of fig. 59. will illuftrat little. Suppofe the curves ace, bkf, clg, &c. to re- CCCcv. prefent 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. Suppofe each fo broad as to project a little over its inferior like the tiles of a roof. It is evident, that 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 dmh along with it. Alfo, because the distance of the middle point o from the axis of motion n p is greater than the distance of m from the axis db, and because will therefore describe a portion of a larger circle than m does, the rib nop will flide up a little under the rib d mh, or the rib dm h will overlap nop a little more than before; the distance om will therefore be diminished. The fame muft happen to all the fuperior ribs; but the change of distance will be lefs and lefs as we go upwards. Now, instead of this great breadth of the ribs overlapping 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 first 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 fecond rib with the first rib contract, the fecond must rife a little, and alfo go outward, and will carry the lower ribs along with it; the third rib will rife ftill farther by the contraction of the muscles which connect it with the fecond, and fo on and thus the whole ribs are raifed and thrown outward (and a little forward, because the articulation of each with the spine is confiderably higher than that with the fternum), and the capacity of the thorax is enlarged by the contrac tion of its mufcular covering. The direction of the muscular fibres is very oblique to the direction of the circular motion which it produces; from which circum ftance it follows, that a very minute contraction of the muscles fure. Effects of mufcles produces all the motion which is neceffary. Air's pref- This indeed is not great; the whole motion of the loweft ribs is less than an inch in the most violent infpiration, and the whole contraction of the muscles of the 12 ribs does not exceed the eighth part of an inch, even fuppofing the intercoftal muscles at right angles to the ribs; and being oblique, the contraction is still lefs (fee BORELLI, SABATIER, MONRO, &c.) It would feem, that the intensity of the contractive power of a mufcular fibre is eafily obtained, but that the space through which it can be exerted is very limited; for in moft cafes nature places the muscles in fituations of great mechanical disadvantage in this respect, in order to procure other conveniences. But this is not the whole effect of the contraction of the intercoftal muscles: fince the compound action of the two sets of muscles, which cross each other from rib to rib like the letter X, is nearly at right angles to the rib, but is oblique to its plane, it tends to push the ribs clofer on their articulations, and thus to prefs out the two pillars on which they are articulated. Thus, fupPlate pofing af (fig. 60.) to reprefent the fection of one of CCCCV. the vertebræ of the fpine, arded a fection 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 nearer to each other, and to prefs the vert ..and the fternum ed 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 prefent purpose. It remains almost fixed atop 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 enlargement of the diameters of the thorax during infpiration is very fmall, not exceeding the fiftieth part of an inch in ordinary cafes. This is eafily calculated. Its quiefcent capacity is about two cubic feet, and we never draw in more than 15 inches. Two fpheres, one of which holds 2 cubic feet and the other 2 feet and 15 inches, will not differ in diameter above the fiftieth part of an inch. The other method of enlarging the capacity of the thorax is very different. It is feparated from the abdomen by a strong mufcular partition called the dia phragm, which is attached to firm parts all around. In its quiefcent or relaxed state it is confiderably convex upwards, that is, towards the thorax, rifing up into its cavity like the bottom of an ordinary quart bottle, only not fo regular in its fhape. Many of its fibres tend from its middle to the circumference, where they are inferted into firm parts of the body. Now fuppofe these fibres to contract. This muft draw down its middle, or make it flatter than before, and thus enlarge the capacity of the thorax. Phyfiologifts are not well agreed as to the share which each of these actions has in the operation of enlarging the thorax. Many refuse all share of it to the intercoftal muscles, and fay that it is performed by the diaphragm alone. But the fact is, that the ribs are really observed to rise even while the person is afleep; and this cannot poffibly be produced by the diaphragm, as thefe anatomifts affert. Such an opinion fhows either ignorance or neglect of the laws of pneumatics. If the capacity of the thorax were enlarged only by drawing down the VOL. XV. Part I. fure. diaphragm, the preffure of the air would comprefe the Effects of ribs, and make them defcend. And the fimple laws Air's prefe of mechanics make it as evident as any propofition in geometry, that the contraction of the intercostal muscles muft produce an elevation of the ribs and enlargement of the thorax; and it is one of the most beautiful contrivances of nature. It depends much on the will of the animal what share each of these actions shall have. In general, the greatest part is done by the diaphragın; and any person can breathe in fuch a manner that his ribs fhall remain motionlefs; and, on the contrary, he can breathe almost entirely by raifing his cheft. In the first method of breathing, the belly rifes during infpiration, because the contraction of the diaphragm compreffes the upper part of the bowels, and therefore fqueezes them outwards; fo that an ignorant perfon would be apt to think that the breathing was performed by the belly, and that the belly is inflated with the air. The ftrait lacing of the women impedes the motion of the ribs, and changes the natural habit of breathing, or brings on an unnatural habit. When the mind is depreffed, it is obferved that the breathing is more performed by the mufcles of the thorax; and a deep figh is always made in this way. Thefe obfervations on the manner in which the capacity of the cheft can be enlarged were neceffary, before we can acquire a juft notion of the way in which the mechanical properties of air operate in applying it to the mafs of blood during its paffage through the lungs. Suppofe the thorax quite empty, and communicating with the external air by means of the trachea or windpipe, it would then resemble a pair of bellows. Raising the boards correfponds to the raifing of the ribs; and we might imitate the action of the diaphragm by forcibly pulling outwards the folded leather which unites them. Thus their capacity is enlarged, and the air rushes in at the nozzle by its weight in the fame manner as water would do. The thorax differs from bellows only in this refpect, that it is filled by the lungs, which is a vaft collection of little bladders, like the holes in a piece of fermented bread, all communicating with the trachea, and many of them with each other. When the cheft is enlarged, the air rushes into them all in the fame manner as into the fingle cavity of an empty tho rax. It cannot be faid with propriety that they are inflated: all that is done is the allowing the air to come in. At the fame time, as their membranous covering muft have fome thickness, however fmall, and fome elafticity, it is not unlikely that, when compreffed by expiration, they tend a little to recover their former fhape, and thus aid the voluntary action of the muscles. It is in this manner that a small bladder of caoutchouc fwells again after compreffion, and fills itself with air or water. But this cannot happen except in the most minute veficles: thofe of fenfible bulk have not elafticity enough for this purpose. The lungs of birds, however, have fome very large bladders, which have a very confiderable elafticity, and recover their shape and fize with great force after compreflion, and thus fill themselves with air. The refpiration of these animals is confiderably different from that of land animals, and their mufcles act chiefly in expiration. This will be explained by and by as a curious variety in the pneu matic inftrument. This account of the manner in which the lungs are U filled fure. 349 our own very But nature has alfo given us a mechanism by which we can expire, namely, the abdominal muscles; and when we have finished an ordinary and eafy expiration, we can ftill expel a confiderable bulk of air (nearly half of the contents of the lungs) by contracting the abdominal muscles. Thefe, by compreffing the body, force up its moveable contents against the diaphragm, and cause it to rife further into the thorax, acting in the fame manner as when we expel the fæces per anum. When a perfon breathes out as much air as he can in this manner, he may obferve that his ribs do not collapse during the whole operation. fure. Effects of filled with air does not feem agreeable to the notions that all that is neceffary for expiration is to cease to Effects of Air's pref- we entertain of it. We feem to fuck in the air; but al- act. No perfon feels any difficulty in emptying the Air' pref though it be true that we act, and exert force, in order lungs; but weak people often feel a difficulty of into get air into our lungs, it is not by our action, but by fpiration, and compare it to the feeling of a weight on We take in external preffure, that it does come in. If we apply their breaft; and expiration is the last motion of the air not by our mouth to the top of a bottle filled with water, we thorax in a dying person. action, but find that no draught, as we call it, of our cheft will by external fuck in any of the water; but if we fuck in the preffure. fame manner at the end of a pipe immersed in water, it follows immediately. Our intereft in the thing makes us connect in imagination our own action with the effect, without thinking on the many fteps which may intervene in the train of natural operations; and we confider the action as the immediate caufe of the air's reception into the lungs. It is as if we opened the door, and took in by the hand a perfon who was really pushed in by the crowd without. If an incifion be made into the fide of the thorax, fo that the air can get in by that way, when the animal acts in the usual manner, the air will really come in by this hole, and fill the space between the lungs and thorax; but no air is fucked into the lungs by this procefs, and the animal is as completely fuffocated as if the windpipe were fhut up. And, on the other hand, if a hole be made into the lungs without communicating with the thorax, the animal will breathe through this hole, though the windpipe be ftopped. This is fuccefsfully performed in cafes of patients whofe trachea is fhut up by accident or by inflammation; only it is neceffary that this perforation be made into a part of the lungs where it may meet with fome of the great pulmonary paffages; for if made into fome remote part of a lobe, the air cannot find its way into the rest of the lungs through fuch narrow paffages, obftructed too by blood, &c. 350 Nature of We have now explained, on pneumatical principles, expiration the process of inspiration. The expiration is chiefly performed by the natural tone of the parts. In the act of infpiration the ribs were raifed and drawn outwards in oppofition to the elafticity of the folids themselves; for although the ribs are articulated at their extremities, the articulations are by no means fuch as to give a free and eafy motion like the joints of the limbs. This is particularly the cafe in the articulations with the fternum, which are by no means fitted for motion. It would feem that the motion really produced here is chiefly by the yielding of the cartilaginous parts and the bending of the rib; when therefore the muscles which produced this effect are allowed to relax, the ribs again collapfe. Perhaps this is affifted a little by the action of the long mufcles which come down across the ribs without being inferted into them. Thefe may draw them together a little, as we comprefs a loose bundle by a ftring. 332 In like manner, when the diaphragm was drawn It requires 352 There feems then to be a certain natural unconftrained A certain ftate of the veficles of the lungs, and a certain quantity quantity of air necefof air neceffary for keeping them of this fize. It is fary to keep probable that this ftate of the lungs gives the freeft mo- the lungs tion to the blood. Were they more compreffed, the of a natural blood veffels would be compressed by the adjoining fize. veficles; were they more lax, the veffels would be more crooked, and by this means obftructed. The frequent infpirations gradually change this air by mixing fresh air with it, and at every expiration carrying off fome of it. In catarrhs and inflammations, especially when attended with fuppuration, the fmall paffages into the remote veffels are obftructed, and thus the renewal of air in them will be prevented. The painful feeling which this occafions caufes us to expel the air with violence, fhutting the windpipe, till we have exerted ftrongly with the abdominal muscles, and made a ftrong compreffion on the lower part of the thorax. We then open the paffage fuddenly, and expel the air and obftructing matter by violent coughing. 353 in birds. We have faid, that birds exhibit a curious variety Process of in the procefs of breathing. The mufcles of their breathing. wings being fo very great, required a very extenfive' infertion, and this is one ufe of the great breaft-bone. Another ufe of it is, to form a firm partition to hinder the action of these muscles from compreffing the thorax in the act of flying: therefore the form of their cheft does not admit of alternate enlargement and contraction to that degree as in land animals. Moreover, the mufcles of their abdomen are also very small; and it would feem that they are not fufficient for producing the compreffion on the bowels which is neceffary for carrying on the procefs of concoction and digeftion.. Inftead of aiding the lungs, they receive help from them. In an oftrich, the lungs confift of a fleshy part A, A (fig. 61.), compofed of veficles like thofe of land ani- Plate mals, and, like theirs, ferving to expofe the blood to CCCCV. the action of the air. Befides thefe, they have on each fide four large bags B, C, D, E, each of which has an orifice G communicating with the trachea; but the fecond, C, has alfo an orifice H, by which it communicates with another bag F fituated below the reft in the abdomen. Now, when the lungs are compreffed by the action of the diaphragm, the air in C is partly expelled by the trachea through the orifice G, and partly driven through the orifice H into the bag F, fure. Effects of which is then allowed to receive it; because the fame Air's pref- action which compreffes the lungs enlarges the abdomen. When the thorax is enlarged, the bag C is partly fupplied with fresh air through the trachea, and partly from the bag F. As the lungs of other animals refemble a common bellows, the lungs of birds refemble the fmith's bellows with a partition; and anatomifts have difcovered paffages from this part of the lungs into their hollow bones and quills. We do not know all the ufes of this contrivance; and only can observe, that this alternate action must affift the mufcles of the abdomen in promoting the motion of the food along the alimentary canal, &c. We can diftinctly observe in birds that their belly dilates when the chest collapfes, and vice versa, contrary to what we fee in the land animals. Another ufe of this double paffage may be to produce a circulation of air in the lungs, by which a compenfation is made for the smaller surface of action on the blood: for the number of small veficles, of equal capacity with these large bags, gives a much more extenfive furface. 354 The operation of fucking, If we try to raise mercury in a pipe by the action of the cheft alone, we cannot raise it above two or three inches; and the attempt is both painful and hazardous. It is painful chiefly in the breaft, and it provokes cough ing. Probably the fluids ooze through the pores of the veficles by the preffure of the furrounding parts. On the other hand, we can by expiration fupport mercury about five or fix inches high: but this alfo is very painful, and apt to produce extravafation of blood. This feems to be done entirely by the abdominal muscles. The operation properly termed SUCKING is totally different from breathing, and resembles exceedingly the action of a common pump. Suppofe a pipe held in the mouth, and its lower end immerfed in water. We fill the mouth with the tongue, bringing it forward, and applying it clofely to the teeth and to the palate; we then draw it back, or bend it downwards (behind) from the palate, thus leaving a void. The preffure of the air on the cheeks immediately depreffes them, and applies them clofe to the gums and teeth; and its preffure on the water in the veffel caufes it to rife through the pipe into the empty part of the mouth, which it quickly fills. We then push forward the tip of the tongue, below the water, to the teeth, and apply it to them all round, the water being above the tongue, which is kept much depreffed. We then apply the tongue to the palate, beginning at the tip, and gradually going backward in this application. By this means the water is gradually forced backward by an operation fimilar to that of the gullet in fwallowing. This is done by contracting the gullet above and relaxing it below, juft as we would empty a gut of its contents by drawing our clofed hand along it. By this operation the mouth is again completely occupied by the tongue, and we are ready for repeating the operation. Thus the mouth and tongue refemble the barrel and pifton of a pump; and the application of the tip of the tongue to the teeth performs the office of the valve at the bottom of the barrel, preventing the return of the water into the pipe. Although ufual, it is not abfolutely neceffary, to withdraw the tip of the tongue, making a void before the tongue. Sucking may be performed by merely feparating the tongue gradually from the palate, beginning at the root. palate, beginning at the root. If we withdraw the tip Effects of of the tongue a very minute quantity, the water gets Air's pref in and flows back above the tongue. The action of the tongue in this operation is very powerful; fome perfons can raife mercury 25 inches: but this ftrong exertion is very fatiguing, and the foft parts are prodigiously swelled by it. It caufes the blood to ooze plentifully through the pores of the tongue, fauces, and palate, in the fame manner as if a cupping-glafs and fyringe were applied to them; and, when the infide of the mouth is excoriated or tender, as is frequent with infants, even a very moderate exertion of this kind is accompanied with extravafation of blood. When children fuck the nurses breast, the milk follows their exertion by the preffure of the air on the breast; and a weak child, or one that withholds its exertions on account of pain from the above-mentioned cause, may be affifted by a gentle preffure of the hand on the breaft: the infant pupil of nature, without any knowledge of pneumatics, frequently helps itfelf by preffing its face to the yielding breaft. In the whole of this operation the breathing is performed through the noftrils; and it is a prodigious diftress to an infant when this paffage is obftructed by mu→ cus. We beg to be forgiven for obferving by the way, that this obftruction may be almost certainly removed, for a little while, by rubbing the child's nofe with any liquid of quick evaporation, or even with water. furc. 355 which is very fimi Olar. The operation in drinking is not very different from And of that in fucking: we have indeed little occafion here drinking, to fuck, but we must do it a little. Dogs and fome other animals cannot drink, but only lap the water into their mouths with their tongue, and then fwallow it. The gallinaceous birds feem to drink very imperfectly; they feem merely to dip their head into the water up to the eyes till their mouth is filled with water, and then holding up the head, it gets into the gullet by its weight, and is then fwallowed. The elephant drinks in a very complicated manner; he dips his trunk into the water, and fills it by making a void in his mouth: this he does in the contrary way to man. After having depreffed his tongue, he begins the application of it to the palate at the root, and by extending the application forward, he expels the air by the mouth which came into it from the trunk. The process here is not very unlike that of the condenfing fyringe without a piston valve, defcribed in n° 58, in which the external air (corresponding here to the air in the trunk) enters by the hole F in the fide, and is expelled through the hole in the end of the barrel; by this operation the trunk is filled with water: then he lifts his trunk out of the water, and bringing it to his mouth, pours the contents into it, and fwallows it. On confidering this operation, it appears that, by the fame procefs by which the air of the trunk is taken into the mouth, the water could also be taken in, to be afterwards fwallowed: but we do not find, upon inquiry, that this is done by the elephant; we have alway's obferved him to drink in the manner now defcribed. In either way is a double operation, and cannot be carried on any way but by alternately fucking and fwallowing, and while one operation is going on the other is interrupted; whereas man can do both at the fame time. Nature feems to delight in exhibiting to rational obfervers her inexhauftible variety of refource; for many infects, which drink with a trunk, drink without interruption: yet we it fure. 356 Mode of Effects of do not call in queftion the truth of the aphorifm, Natura Air's pref- maxime fimplex et femper fibi confona, nor doubt but that, if the whole of her purpose were feen, we should find that her procefs is the fimpleft poffible: for Nature, or Nature's God, is wife above our wifeft thoughts, and fimplicity is certainly the choice of wisdom: but alas! it is generally but a small and the most obvious part of her purpofe that we can obferve or appreciate. We seldom fee this fimplicity of nature ftated to us, except by fome system-maker, who has found a principle which fomehow tallies with a confiderable variety of phenomena, and then cries out, Fruftra fit per plura quod fieri poteft per pauciora. There is an operation fimilar to that of the elephant, keeping up which many find a great difficulty in acquiring, viz. a continued keeping up a continued blaft with a blow-pipe. We blaft with a would defire our chemical reader to attend minutely to blow-pipe. the gradual action of his tongue in fucking, and he will find it fuch as we have defcribed. Let him attend particularly to the way in which the tip of the tongue performs the office of a valve, preventing the return of the water into the pipe: the fame pofition of the tongue would hinder air from coming into the mouth. Next let him obferve, that in fwallowing what water he has now got lodged above his tongue, he continues the tip of the tongue applied to the teeth; now let him shut his mouth, keeping his lips firm together, the tip of the tongue at the teeth, and the whole tongue forcibly kept at a distance from the palate; bring up the tongue to the palate, and allow the tip to feparate a little from the teeth; this will expel the air into the space between the fauces and cheeks, and will blow up the cheeks a little then, acting with the tip of the tongue as a valve, hinder this air from getting back, and depreffing the tongue again, more air (from the noftrils) will get into the mouth, which may be expelled into the fpace without the teeth as before, and the cheeks will be more inflated: continue this operation, and the lips will no longer be able to retain it, and it will ooze through as long as the operation is continued. When this has become familiar and eafy, take the blow-pipe, and there will be no difficulty in maintaining a blast as uniform as a fmith's bellows, breathing all the while through the noftrils. The only difficulty is the holding the pipe: this fatigues the lips; but it may be removed by giving the pipe a convenient shape, a pretty flat oval, and wrapping it round with leather or thread. Another phenomenon depending on the principles already established, is the land and fea-breeze in the warm countries. We have feen that air expands exceedingly by heat; therefore heated air, being lighter than an equal bulk of cold air, must rife in it. If we lay a hot ftone in the funshine in a room, we fhall obferve the fhadow of the ftone furrounded with a fluttering fhadows of different degrees of brightnefs, and that this flutter rifes rapidly in a column above the ftone. If we hold an extinguished candle near the ftone, we fhall fee the smoke move towards the ftone, and then afcend up from it. Now, suppose an island receiving the first rays of the fun in a perfectly calm morning; the ground will foon be warmed, and will warm the contiguous air. If the island be mountainous, this effect will be more remarkable; because the inclined fides of the hills will receive the light more directly: the midland air will therefore be moft fure. warmed: the heated air will rife, and that in the Effects of middle will rife fafteft; and thus a current of air upwards Air's pref. will begin, which must be supplied by air coming in from all fides, to be heated and to rife in its turn; and thus the morning fea-breeze is produced, and continues all day. This current will frequently be reverfed during the night, by the air cooling and gliding down the fides of the hills, and we shall have the land-breeze. 358 mines. It is owing to the fame cause that we have a circula- Circulation tion of air in mines which have the mouths of their of air in fhafts of unequal heights. The temperature underground is pretty conftant through the whole year, while that of the atmosphere is extremely variable. Now, fuppofe a mine having a long horizontal drift, communicating between two pits or fhafts, and that one of these shafts terminates in a valley, while the other opens on the brow of a hill perhaps 100 feet higher. Let us further fuppofe it fummer, and the air heated to 65°, while the temperature of the earth is but 45°; this last will be alfo the temperature of the air in the fhafts and the drift. Now, fince air expands nearly 24 parts in 10000 by one degree of heat, we shall have an odds of preffure at the bottom of the two fhafts equal to nearly the 20th part of the weight of a column of air 100 feet high (100 feet being fuppofed the difference of the heights of the fhafts). This will be about fix ounces on every square foot of the fection of the fhaft. If this preffure could be continued, it would produce a prodigious current of air down the long fhaft, along the drift, and up the short shaft. The weight of the air acting through 100 feet would communicate to it the velocity of 80 feet per fecond: divide this by 20, that is, by 4,5, and we fhall have 18 feet per fecond for the velocity: this is the velocity of what is called a brisk gale. This preffure would be continued, if the warm air which enters the long shaft were cooled and condensed as fast as it comes in; but this is not the cafe. It is however cooled and condenfed, and a current is produced fufficient to make an abundant circulation of air along the whole paffage; and care is taken to difpofe the fhafts and conduct the paffages in fuch a manner that no part of the mine is out of the circle. When any new lateral drift is made, the renewal of air at its extremity becomes more imperfect as it advances; and when it is carried a certain length, the air ftagnates and becomes fuffocating, till either a communication can be made with the reft of the mine, or a fhaft be made at the end of this drift. As this current depends entirely on the difference of temperature between the air below and that above, it muft ceafe when this difference ceafes. Accordingly, in the fpring and autumn, the miners complain much of ftagnation; but in fummer they never want a current from the deep pits to the fhallow, nor in the winter a current from the fhallow pits to the deep ones. It frequently happens alfo, that in mineral countries the chemical changes which are going on in different parts of the earth make differences of temperature fufficient to produce a fenfible current. It is easy to fee that the fame causes must produce a current down our chimneys in fummer. The chimney is colder than the fummer air, and must therefore condenfe it, and it will come down and run out at the doors and windows. 359 The nature of what is And this naturally leads us to confider a very impor- called the tant effect of the expanfion and confequent afcent of air draught in by chimneys. |