A I R
A I R
bubble, more or lefs compreffed according to the depth the fifh fwims at, and takes up more or lefs fpace j and conse- quently the body of the fifh, part of whole bulk this bladder is, is greater or lefs according to the feveral depths, tho' it retains the fame weight- The rule of hydroftatics is, that a body heavier than Co much water, as is equal in quantity to the bulk of it, will fink 3 a body lighter, will fwim : a body of equal weight will reft in any part of the water. By which rule if the fifli in the middle region of the water, be of equal weight with an equal bulk of the water, the fifti will reft there, without any tendency either upwards or down- wards i and if the fifh be deeper in the water, its bulk be- coming lefs by the comprcflion of the bladder, and yet retaining 'the fame weight, it will fink and reft at the bottom : on the other fide, if the fifh be higher than the middle region, the air dilating itfelf, and the bulk of the fifli confequently in- •creafing, without any increafe of the weight, the fifli will rife and reft at the top of the water.
Perhaps the fifh by fome action can emit air out of the bladder, and afterwards out of its body ; and alfo when there is not enough, take in more air, and convey it to this bladder 3 in which cafe it will be no wonder, that there fhould be always a fit proportion of air in the bodies of all fifties, to ferve their ufe according to the depth of water they live in : per- haps alfo by fome mufcle, the fifh can contrail this bladder be- yond the prefTure of the weight of water ; and perhaps it can by its fides, or fome other defence, keep oft" the preflure of the water, and give the air leave to dilate itfelf. In thefe cafes, the fifh* will be helped in all intermediate diftances, and may rife or fink from any region without moving a fin. Phil. Tranf.
N°.
1 14. p. 310.
If the Air-bladder of a fifli be pricked or broken, the fifli prefently finks to the bottom, unable either to fupport or raife itfelf up again. Flat fifties, as foles, plaife, c?V. which always lie groveling at the bottom, having no Air-bladder. In moft fifties there is a manifeft channel, leading from the gullet, or upper orifice of the ftomach to the Air-bladder ', which doubtlefs ferves for conveying air into it. In a ftur- gcon Mr. Wilhighby obferved, that upon preffing the blad- der, the ftomach prefently fwelled ; fo in that fifh it feems the air pafles freely both ways. PofTibly the fifli while alive may have a power to raife up this valve, and let out air on occafion.
All the cartilaginous kind want Air-bladders : by what means it is they afcend and defcend in water, is yet unknown. The cetaceous kind, or fea beafts, are alfo without the Air-bladder ; indeed, as thefe differ in little from quadrupeds, but the want of feet ; the air which they receive into their lungs in infpi- ration, may ferve to render their bodies equiponderant to wa- ter ; and the conftridtion or dilatation of it, by the help of the diaphragm, and mufcles of refpiration, may poflibly aflift them to afcend, or defcend in the water, by a light impulfe thereof with their fins 3 . Moft of the eel kind have bladders, yet they can hardly raife themfelvcs in the water, by reafon of the length and weight of their tails : the Air-bladder being near their heads, may help them to lift up their heads and fore part b .— [* Ray, Wifd. of God in Great. P. 1. p. 26. b Phil. Tranf. N°. 115. p. 349. feq.]
There is great diverlity in Air-bladders, in refpecf. of figure, fubftance, fituation, and connexion, in different fifties. In fome, the Air-bladder is divided into two, e. gr. in carps 3 and in others, into three. Needham maintains, that all fifties which have teeth in their jaws, have only a fingle Air -bladder ; whereas thofe without teeth have a double one, or which amounts to the fame, the Air-bladder of thefe laft is divided into two cells. Sig. Redi refutes this diftinction ; giving in- stances of fifties with teeth, whole Air-bladder is double ; and of others without teeth, who have only a fingle Air-bladder. V. Nouv. Rep. Lett. T. 43. p. 282. feq. The water fnake in lieu of a bladder, has a large membran- ous air-bag on its back, which it empties and fills with air at pleafure, by an appcrture, which it can ftiut very clofe, from without inwards, by means of a fort of valve, fo that the leaft globule of water cannot enter without its confent. By this artifice it can enlarge or lefle'n the bulk of its body, and inhabit all depths of the water c . Tho' a conjecture has been advanced by Mr. Ray, that it is by the help of water which they take in, and let out, by two holes in the lower part of their abdomen, near to the ventricle. They fink in the water, by letting in fome of it, at thefe holes ; the orifices whereof are opened and fhut at plcafure, by means of proper niufcles. The water being thus received into the cavity of their abdomen, by which they preponderate the water, and defcend. When they would afcend again, a compreflion is made by the mufcles of their abdomen, and the water forced out again, at leaft fo much as fuffices to give the degree of levity wanted d .— [ c V. Jour, des Scav. T. 22. p. 264. d Ray, Wifd. of God. P. 2. p. 346.]
AIR-Gun. See Wind-Gun.
AIR-Shafts, among miners, denote holes or {hafts let down from the day to meet the adits, and furnifh frefli air. The damps, want and impurity of air, which occur when adits are wrought 30 or 40 fathom long, make it necefFary to let down Air-Shafts, in order to give the air liberty to play thro'
the whole work, and thus difcharge bad vapours, and furnifli good air for refpiration : the expehce of which {hafts in re- gard of their vaft depths, hardnefs of the rock, drawing of water, €£f*r, fomctimes equals, nay exceeds the ordinary charge of the whole adit.
Sir Robert Murray defcribes a method, ufed in the coal mines at Liege, of working mines, without Air-Jb'afts. Phil. Tranf. Nf.5.
When the miners at Mendip have funk a groove, they will not be at the charge of an Air-jhaft, till they come at ore j and for the fupply of air have boxes of elm exactly clofed, cf about fix inches in the clear, by which they carry it down above twenty fathom. They cut a trench at a little diftance from the top of the groove, covering it with turf and rods difpofed to receive the pipe, which they contrive to come in fide-ways to their groove, four feet from the top ; which carries down the air to a great depth. -When they come at ore, and need an Air-Jlmft, they fink it four or five fathom diftant, according to the convenience of the breadth, and of the fame faihion with the groove, to draw as well ore as air. Phil. Tranf. N°. 39. p. 769.
AIR-Threads of fpiders. See Air-Threads.
AIR-Vessels in plants, are certain canals, or ducts, whereby a kind of refpiration is effected in vegetable bodies. See Plant, Respiration, csY.
Air-Vcjfels ftand diftinguiflicd from fap-veflels ; the former an- fwering to the trachea, and lungs of animals, the latter to their lacteals, and blood-veffels.
Dr. Grew has an inquiry into the motion and caufe of the air in vegetables ; wherein he {hews that it enters them various ways, not only by the trunk, leaves, and other parts above ground, but at the root. For the reception, as weil as ex- pulfion of air, the pores are fo very large in the trunks of" fome plants, as in the better fort of thick walking canes, that they are vifible to a good eye without a glafs j but with a glafs, the cane feems as if it were ftuck full of large pin- holes 3 refembling the pores of the fkin in the ends of the fin- gers, and ball of the hand. In the leaves of the pine, thro* a glafs they make an elegant fhow 3 ftanding almoft exactly in rank and file, throughout the length of the leaves. But tho' the air enters in part at the trunk and other parts, es- pecially in fome plants ; yet its chief admiflion is at the root ; much as in animals, fome parts of the air may continually pafs into the body and blood by the pores of the fkin 3 but the chief draught is at the mouth. If the chief entrance of the air were at the trunk ; before it could be mixed with the fap in the root, it muft defcend 3 and fo move not only contrary to its own nature, but in a contrary courfe fo the fap : where- as by its reception at the root, and its tranfition from thence, it has a more natural and eafy motion of afcent. The fame is farther argued, from the finenefs and fmallnefs of the diametral apertures in the trunk, in companion of thofe in the root j which nature has plainly defigned for the Separation of the air from the fap, after they are both together received thereinto. Grew, Anat. of Root. c. 3. p. 127. Air-Vejfeh are found in the leaves of all plants, and are even difcoverable in many without the help of glaffes : for upon breaking the ftalk, or chief fibres of a leaf; the Iikenefs of a fine woolly fubftance, or rather of curious finall cob-webs, may be feen to hang at both the broken ends. This is taken notice of only in fome few plants, as in fcabious, where it is more vifible : but may alfo be feen more or lefs in moft others, if the leaves be very tenderly broken. This wool is really a fkein of Air-vejfels, or rather of the fibres of the Air-vefjels, Ioofed from their fpiral pofition, and fo drawn drawn out in length. Id. ibid. c. 4. p. 155.
That air is infpired by vegetables, has been fully proved by Mr. Hales, in his ftatical cflays; and he has in many inftances fliewn, that air freely enters the veffels of trees, and that it is in great abundance wrought into their fubftance. But as to particular Air-vejfels in plants, he feems to fpeak doubt- fully ; he fays by way of queftion, may not the ufe of thofe fpiral wreaths, that are coiled round the infides of thofe vef- fels, which are fuppefed to be Air-vefj'els, and which are mani- feftly to be feen in feveral trees ; as alio in the leaves of the vine and fcabious, may not thefe be defigned by nature to promote the quicker afcent of Air, by being in fome mea- fure conformed to its claftic contortions ? For fuch fpiral wreaths feem to be altogether ufelefs, for promoting the afcent of any liquor, as the fap, which afcends moft freely thro' in- numerable other capillary veflels, having no fuch fpiral coils in them. Not that we are to fuppofe the air in its elaftic ftate actually to touch, and thereby to be determined in the courfe of thefe fpirals, as any liquor would be. But as the rays of light, when they are reflected from a folid body, are found to be reflected, without actually touching the re- flecting body in the point of reflection 3 fo it is not unreason- able to fuppofe, that elaftic air may, like light, be diverted from one courfe, and fo be determined to another, by the folid bodies it approaches without touching them, but rebounding like light from thofe folid bodies near the point of contact. Mr. Hales has obferved, that thefe fpirals are coiled in a courfe oppofite to the courfe of the fun, that is, from Weft to Eaft, V. Static. Eft", Vol. 2. p. 265, 266.
AIRA-
