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to the circumference, where they are inferted into firm parts of the body. Now, fuppofe these Ebres to contract. This muft draw down its middle, or make it flatter than before, and thus calarge the capacity of the thorax.

Phyologifts differ as to the fhare which each of these actions has in enlarging the thorax. Many dey all hare of it to the intercoftal muscles, and fy it is performed by the diaphragm alone. But the fact is, that the ribs are really observed to rife eves during fleep; and this cannot be produced by the diaphragm. If the capacity of the thorax were enlarged only by drawing down the dia phragm, the preffure of the air would comprefs the ribs, and make them defcend. The laws of mechanics make it evident, that the contraction of the intercoftal muscles must produce an elevafin of the ribs and enlargement of the thorax; and it is one of the most beautiful contrivances of satare. It depends much on the will of the animal what share each of these actions shall have. In general, the greateft part is done by the diaphragm; and any perfon can breathe in fuch a manner that his ribs fhall remain motionless; and, the contrary, he can breathe almost entirely by mailing his cheft. In the first method of breathing, the belly rifes during inspiration, becase the contraction of the diaphragm compreffes the upper part of the bowels, and therefore zes them outwards. When the mind is deped, the breathing is more performed by the es of the thorax; and a deep figh is always make in this way.

the

ples, of the procefs of infpiration. The expiration is chiefly performed by the natural tone of the parts. In the act of inspiration the ribs were raifed and drawn outwards in oppofition to the elafticity of the folids themfelves; 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 not fitted for motion. The motion really produced here is chiefly by the yielding of the cartilaginous parts, and the bending of the ribs: when therefore the mufcles which produced this effect are allowed to relax, the ribs again collapfe. In like manner, when the diaphragm was drawn down, it compressed the abdomen in oppofition to the elafticity of all the viscera contained in it, and to the elasticity and tone of the teguments and mufcles which furround it. When therefore the diaphragm is relaxed, these parts push it up again into its natural fituation, and in doing this expel the air from the lungs.

On this account expiration is performed without any effort. We feel that, after having made an ordinary eafy infpiration, it requires the con tinuance of the effort to keep the thorax in this enlarged ftate, and that all that is neceflary for expiration is to ceafe to act. No perfon feels any difficulty in emptying the lungs; but weak people often feel a difficulty of infpiration, and compare it to the feeling of a weight on their breaft: and expiration is the laft motion of the thorax in a dying perfon.

But we have alfo a mechanifm by which we can expire, namely, the abdominal muscles; and when we have finished an eafy expiration, we can fill expel a confiderable bulk of air (nearly half the contents of the lungs) by contracting the abdominal mufcles. Thefe, by compreffing the body, force up its moveable contents against the diaphragm, and caufe it to rife further into the thorax. When a perfon breathes out as much air as he can in this manner, he may observe that his ribs do not collapfe during the whole opera

tion.

Thefe obfervations, on the manner in which the capacity of the cheft can be enlarged, are necessary to give a juft idea of the way in which the mechanical properties of air operate, in applying to the mafs of blood during its paffage through the lungs, which have been compared to bellows. The thorax differs from bellows only in this reped, 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 thorax. They cannot be faid It appears then, that there is a certain natural with propriety to be inflated: all that is done is unconstrained ftate of the veficles of the lungs, allowing the air to come in. At the fame and a certain quantity of air neceflary for keeptime, as their membraneous covering muft have ing them of this fize. This ftate of the lungs fome thickness, however fmall, and fome elaftici- probably gives the freeft motion to the blood. ty, it is not unlikely that, when compreffed by Were they more compreffed, the blood veffels espiration, they tend to recover their former would be compreffed by the adjoining veficles; fape, and thus aid the voluntary action of the were they lefs fo, the veffels would be more mufcles. Thus a fmall bladder of caoutchone crooked, and thus obftructed. The frequent inwells again after compreffion, and fills itfelf with fpirations gradually change this air by mixing ar. But this cannot happen except in the most fresh air with it, and at every expiration carrying nute veficles: thofe of fenfible bulk have not off fome of it. In catarrhs and inflammations, aficity enough for this purpofe. The lungs of especially when attended with fuppuration, the birds, however, have fome very large bladders, fmall paffages into the remote veffels are obftructwhich have a confiderable elafticity, and recover ed, and thus the renewal of air in them will be preffion, and thus fill themfelves with air. The lions caufes us to expel the air with violence, tair fhape and fize with great force after com- prevented. The painful feeling which this occarepiration of

a chiefly in expiration.

fet from that of land animals, and their mufcles Such is the explanation, on pneumatic princi

VOL. XVIII. PART I.

ftrongly with the abdominal mufcles, and made a ftrong compreffion on the lower part of the thorax. We then open the paffage fuddenly, and

E

expel

expel the air and obftructing matter by violent coughing.

BIRDS, as above hinted, exhibit a curious va riety in the process of breathing. The mufcles of their wings being fo very great, require 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 hincer the action of thefe muicles from compreffing the thorax in the act of Aying: there fore the form of their cheft does not admit of alternate enlargement and contraction to the degree as in land animals. The mufcies of their abdomen

are alfo very fail; and they are not fafficient for producing the compreffion on the bowels which is neceflary for carrying on the process of concoction and digeftion. Instead of aiding the lungs, they receive help from them.

The longs of an offici confist of a fleshy part A, A g. 66. Plate 282.), compofed of veficles like thofe of land animal, and, like theirs, ferving to expofe the blood to the action of the air. Beides 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 also an orince H, by which it communicates with another bag F fituated below the relt in the abdomen. 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, which is then allowed to receive it; becaufe the fame 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 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. This alternate action muft affift the mufcles of the abdomen in promoting the motion of the food along the alimentary canal, &c. In birds, their belly dilates when the cheft collapfes, and vice verfa, contrary to what we fee in the land animals. This double paffage may alfo produce a circulation of air in the lungs, to compenfate for the fmaller furface of action on the blood: for the number of fmall veficles, of equal capacity with there large bags, gives a much more extenfive furface.

When we try to raife mercury in a pipe by the action of the cheft alone, we cannot raife it above 2 or 3 inches; and the attempt is both painful and hazardous. But we can by expiration fupport mercury about 5 or 6 inches high, though this alfo is very painful, and apt to produce extravafation of blood. This feems to be done entirely by the abdominal muscles.

The act of SUCKING is totally different from breathing, and refembles 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 (be bind) from the palate, thus leaving a void. The preffure of the air on the cheeks immediately de

preffes them, and applies them close to the gur
and teeth; and its preffure on the water in th
vellel caufer it to rife through the pipe into th
empty part of the mouth, which it quickly fil
We then push forward the tip of the tongue, b
low the water, to the teeth, and apply it to the
all rourd, the water being above the tongu
which is kept much deprefied. We then app
the tongue to the palate, beginning at the ti
and gradually going backward in this applicatio
The water is thus gradually forced backward
an operation fimilar to that of the gullet in fwa
lowing. This is done by contracting the gull
above, and relaxing it below, whereby the mou
is again completely occupied by the tongue, an
we are ready for repeating the operation. The
the mouth and tongue refemble the barrel az
pifton of a pump; and the application of the t
of the tongue to the teeth performs the office
the valve at the bottom of the barrel, prevents
the return of the water into the pipe. Althoug
ufual, it is not abfolutely neceffary, to withdra
the tip of the tongue, making a void before i
Sucking may be performed by merely feparati
the tongue gradually from the palate, beginnin
at the root. If we withdraw the tip of the tong
a very minute quantity, the water gets in an
flows back above the tongue.

The action of the tongue in this operation very powerful; fome perfons can raife mercur 25 inches: but this ftrong exertion is very fatiga ing, and the foft parts are prodigioufly fwelk by it. It caufes the blood to ooze plentiful through the pores of the tongue, fauces, and r late, as if a cupping-glafs and fyringe were applie to them; and, when the infide of the mouth excoriated or tender, as is frequent with infart even a very moderate exertion of this kind is ad companied with extravafation of blood. Whe children fuck the nurfes breaft, the milk follow their exertion by the preffure of the air on th breaft; and a weak child, or one that withhold its exertion on account of pain from the abov mentioned caufe, may be affifted by a gent preffure of the hand on the breaft: the infan rupil of nature, without any knowledge of pne matics, frequently helps itfelf by prefling its fac to the yielding breaft. In the whole of this op ration the breathing is performed through th noftrils; and it is a prodigious diftrefs to an fant when this paffage is obftructed by mucus This obftruction may be almoft certainly rem ved, by rubbing the child's nofe with water or a liquid.

The act of drinking is not very different fret that of fucking: we have indeed little occafic here to fuck, but we muft do it a little. Deg and fome other animals cannot drink, but ou lap the water into their mouths with their tongue and then fwallow it. The gallinaceous birds dr very imperfectly; they merely dip their head i to the water up to their eyes, till their mouth filled with water, and then holding up the he it gets into the gullet by its weight, and is f lowed. The elephant drinks in a very comp cated manner; he dips his trunk into the water and fills it by making a void in his mouth, in th contrary way to man. After having depreff

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 procela bere is like that of the condensing fyringe without a pifton valve, defcribed above in Sect. I. which the external air (correfponding here to the air in the trunk) enters by the hole F in the fe, 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. It is a double operation, and cannot be carried on any way but by alternately fucking and fwallowing; whereas man can do both at the fame time. Many infects, too, which drink with a trunk, drink without in teruption.

There is an operation fimilar to that of the elephant, which young chemifts and jewellers find a great difficulty in acquiring, viz. keeping up a continued blast with a blow-pipe. We would deûre our chemical reader to attend minutely to the gradual action of his tongue in fucking, and he will find it fuch as we have defcribed. Let in attend particularly to the way in which the top of the tongue performs the office of a valve. preventing the return of the water into the pipe: the fame pofition of the tongue will hinder air from coming into the mouth. Next let him obfest, that in fwallowing what water he has now edged above his tongue, he continues the up the tongue applied to the teeth; now let hut his mouth, keeping his lips firm toge ther, 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 w the tip to feparate a little from the teeth; this will expel the air into the space between the faces and cheeks, and will blow up the cheeks alittle; then acting with the tip of the tongue as arave, hinder this air from getting back, and depreffing the tongue again, more air (from the adrils) will get into the mouth, which may be expeed into the fpace without the teeth as before, and the cheeks will be more inflated: confine this operation, and the lips will no longer be able to retain it, and it will ooze through as as the operation is continued. When this tis become familiar and eafy, take the blow-pipe, and there will be no difficulty in maintaining a bal as uniform as a fmith's bellows, breathing all he wine through the noftrils. The only difficul ty is the holding the pipe: this fatigues the lips; but it may be removed by giving the pipe a conFesent fhape, 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 the warm countries. As air expands exceedgy by heat, heated air, being lighter than an equal bulk of cold air, muft rife in it. Suppofe an if and receiving the first ray's of the fun in a calm ording; the ground will foon be warmed, and warm the contiguous air. If the island be ountainous, the inclined fides of the hills will eive the light more directly; the midland air will therefore be moft warmed: the heated air

will rife, and that in the middle will rife fastest; and thus a current of air upwards will begin, which must be fupplied 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 then we thall have the land-breeze.

From the fame caufe there is a circulation of air in mines which have the mouths of their fhafts of unequal heights. The temperature under ground is pretty conflant through the year, while that of the atmosphere is extremely variable. Suppose a mine having a long horizontal drift, communicating between two pits or fhafts, and that one of these terminates in a valley, while the other opens on the brow of a hill perhaps 100 feet higher. Suppofe it fummer, and the air heated to 65°, while the temperature of the earth is but 45; this laft 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 fhall 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 fquare 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 fhort fhaft. This preffure would be continued, if the warm air which enters the long fhaft were cooled and condenfed as faft as it comes in ; but this is not the cafe. It is however cooled and condensed, and a current is produced fufficient to make an abundant circulation of air along the whole paffage; and care is taken to difpofe the shafts 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 fagnates 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 on the difference of temperature between the air below and that above, it must ceafe when this difference ceafes. Accordingly, in the fpring and autumn, the miners complain much of flagnation; but in fummer they never want a current from the deep pits to the thallow, nor in winter a current from the fhallow pits to the deep ones. It often happens also, that the chemical changes which are going on in different parts of the carth make differences of

temperature fufficient to produce a sensible cur. rent.

The fame caufes muft produce a current down our chimneys in fummer. The chimney is colder than the fummer air, and muft therefore condense it, and it will come down and run out at the doors and windows. This leads us to confider a very important effect of the expanfion and confequent afcent of air by heat, namely the draaving of chimneys. The air which has contributed to the burning of fuel must be intenfely heated, and will

E 2

rife

poffible with the fire, or even to pass through it, and this in fuch quantities as juft to confume a quantity of fuel fufficient for producing the heat required; and the ftove must be fo conftructed, that both the burning fuel and the air which has ber heated by it shall be applied to as extenfive farface as poffible of furnace, all in contact with the air of the room; and the heated air within the Love muft not be allowed to get into the funnel which is to carry it off, till it is too much cooled to produce any confiderable heat on the outfide

of the flove.

Our neighbours on the continent, and efpecially towards the north, where the cold of winter is intenfe, and fuel very dear, have beftowed much ation on the conftruction of stoves, and have biced ingenious economy with every elegance of form. Nothing can be handfomer than the fieres of Fayencerie that are to be feen in French Flanders, or the Ruffian ftoves at St Petersburg, faded in ftucco. Here, however, we fhall only confider a ftove in general as a fubject of pneumacal difcuffion, and refer to the article STOVE for a farther account of them.

The general form of a ftove, and of which all there are only modifications adapted to circum. Ences of utility or tafte, is as follows. MIKL (fig. 67, pl. CCLXXXI.) is a quadrangular box of any fize in the direction MI, IK. The infide width from front to back is never lefs than to inches, and rarely extends to 20; the indated fpace is divided by partitions. The loweft chamber AB is the receptacle for the fuel, which es on the bottom of the stove without any grate: this fire-place has a door AO turning on hinges, and in this door is a very fmall wicket P: the roof of the fire-place extends to within a very few inches of the farther end, leaving a narrow paffage B for the flame. The next partition C is about 8 inches higher, and reaches almoft to the other end, leaving a narrow paffage for the flame C. The partitions D d, Ee, Ff Gg, Hb, are repeated above, at the diftance of 8 inches, leaving pallages at the ends, alternately difpofed as in the figure; the laft of them, H, communicates with the room vent. This communication may be regulated by a plate of iron, which can be flid acrofs it by a rod or handle which comes through the fide. The more ufual way of fhutting up this paffage is by a fort of pan or bowl of earthen ware, which is whelmed over it with its brim refing in fand contained in a groove formed all round the hole. This damper is introduced by a door in the front, which is then fhut. The whole is fet on low pillars, fo that its bottom may be a few inches from the floor of the room: it is ufually placed in a corner, and the apartments are fo difpofed that their chimneys can be joined in ftacks as with us.

Some ftraw or fhavings of wood are firft burnt on the hearth at its farther end. This warms the air in the ftove, and creates a determined current. The fuel is then laid on the hearth close by the door, and pretty much piled up. It is now kindied; and the current being directed to the vent, there is no danger of any smoke coming out into the room. To prevent this, the door is fhut, and the wicket P opened. The air fupplied by this,

being directed to the middle or bottom of the fuel, quickly kindles it, and the combuftion goes on.

The intention of this conftruction is obvious. The flame and heated air are retained as long as poffible within the flove by means of the long paffages; and the narrowness of thefe forces the flame into contact with every particle of foot, so as to confume it completely, and thus convert the whole combuftible matter of the fuel into heat. For want of this a very confiderable portion of cur fuel is wafted by our open fires, even under the very best management: the foot which flicks to our vents is very inflammable, and a pound weight of it will give as much, if not more heat, than a pound of coal. And what flicks to our vents is very inconfiderable, in comparifon with what efcapes unconfumed at the chimney top. In fires of green wood, peat, and fome kinds of pit-coal, nearly one fifth of the fuel is loft in this way: but in thefe ftoves there is hardly ever any mark of foot to be feen; and even this finall quantity is produced only after lighting the fires. The volatile inflammable matters are expelled from partз much heated indeed, but not fo hot as to burn; and fome of it charred or half burnt cannot be any further confumed, being enveloped in flame and air already vitiated and unfit for combustion. But when the ftove is well heated, and the current brisk, no part of the foot escapes the action of the air.

The hot air thus retained in the body of the ftove is applied to its fides in a very extended furface. To increase this ftill more, the ftove is made narrower from front to back in its upper part; a certain breadth is neceflary below, for fuel. By diminishing their breadth, the proportion of ufeful furface is increafed. It is with the fame view of making an extenfive application of a hot furface to the air, that the ftove is not built in the wall, nor even in contact with it, nor with the floor: for by its detached fituation, the air in contact with the back, and the bottom (where it is hotteft), is warmed, and contributes one half of the effect; for the great heat of the bottom makes its effect on the air of the room at least equal to that of the two ends. Sometimes a ftove makes part of the wall between two fmall rooms.

On the whole, the effect of a ftove depends much on keeping in the room the air already heated by it. A fmall open fire in the fame room will be fo far from increafing its heat, that it will greatly diminifh it; it will even draw the warm air from a fuite of adjoining apartments. This is obferved in the houfes of the English merchants in St Petersburgh: their habits of life in Britain make them uneafy without an open fire in their rooms; and this obliges them to heat their ftoves twice a day, and their houfes are cooler than those of the Ruffians, who heat them only once. In many German houfes, especially of the lower clafs, the fire-place of the ftove does not open into the room, but into the yard or a lobby, where all the fires are lighted and tended; whereby is avoided the expence of warm air which muft have been carried off by the flove; but this must be equally unpleafant and unwholefome.

The directions refpecting ftoves and their conftruction will equally apply to hot walls in gar

dening;