Air pump. delicate valve. This allows the air to efcape, and fhuts again as foon as the pifton begins to defcend, leaving almost a perfect void behind it as before. Plate 133 ton's pump. This pump has another advantage. It may be changed in a moment from a rarefying to a condensing engine, by fimply turning the cocks at Q and T. While T communicates with the open air and Q with the receiver, it is a rarefying engine or air-pump: but when T communicates with the receiver, and Qwith the open air, it is a condenfing engine. Fig. 23. reprefents Mr Smeaton's air-pump as it is CCCCII. ufually made by Nairne. Upon a folid bafe or taDefcription ble are fet up three pillars F, H, H: the pillar F of Smea fupports the pump-plate A; and the pillars H, H, fupport the front or head, containing a brafs cog-wheel, which is turned by the handle B, and works in the rack C faftened to the upper end of the piston rod. The whole is ftill farther steadyed by two pieces of brass cb and ok, which connect the pump-plate with the front, and have perforations communicating between the hole a in the middle of the plate and the barrel, as will be defcribed immediately. DE is the barrel of the pump, firmly fixed to the table by screws thro' its upper flanch: efde is a fender brafs tube screwed to the bottom of the barrel, and to the under hole of the horizontal canal cb. In this canal there is a cock which opens a communication between the barrel and the receiver, when the key is in the pofition reprefented here: but when the key is at right angles with this pofition, this communication is cut off. If that fide of the key which is here drawn next to the pump-plate be turned outward, the external air is admitted into the receiver; but if turned inwards, the air is admitted into the barrel. gb is another flender brafs pipe, leading from the difcharging valve at g to the horizontal canal bk, to the under fide of which it is fcrewed faft. In this horizontal canal there is a cock n which opens a paffage from the barrel to the receiver when the key is in the pofition here drawn ; but opens a paffage from the barrel to the external air when the key is turned outwards, and from the receiver to the external air when the key is turned inwards. This communication with the external air is not immediate but through a fort of box i; the use of this box is to receive the oil which is discharged through the top valve g.. In order to keep the pump tight, and in working order, it is proper fometimes to pour a tablefpoonful of olive-oil into the hole a of the pump-plate, and then to work the pump. The oil goes along the conduit bedfe, gets into the barrel and through the pifton-valve, when the pifton is preffed to the bottom of the barrel, and is then drawn up, and forced through the difcharging valve g along the pipe gh, the horizontal paffage bn, and finally into the box i. This box has a fmall hole in its fide near the top, through which the air efcapes. From the upper fide of the canal eb there rifes aflender pipe which bends outward and then turns downwards, and is joined to a small box, which cannot be feen in this view. From the bottom of this box proceeds downwards the gage-pipe of glass, which enters the ciftern of mercury G fixed below. On the upper fide of the other canal at o is feen a fmall ftud, having a fhort pipe of glafs projecting horizontally from it, clofe by and parallel to the front piece of the pump, and reaching to the other canal. This Plate Plate CCCCLL The piece hip, in fig. 22, no 2. is the fame with the little cylinder obfervable on the upper fide of the hori. CCCCL zontal canal cd, in fig. 23. The upper part hi is formed into an outside screw, to fit the hollow screw of the piece deed. The top of this last piece has a hole in its middle, giving an eafy paffage to the bent tube cba, fo as to flip along it with freedom. To the end c of this bent tube is foldered a piece of brass cƒg, perforated in continuation of the tube, and having its end. ground flat on the top of the piece hip, and alfo covered with a flip of thin leather strained across it and pierced with a hole in the middle. It is plain from this form, that if the furface fg be applied to the top of hi, and the cover deed be fcrewed down on it, it will draw or prefs them together, fo that no air can escape by the joint, and this without turning the whole tube cbe round, as is neceflary in the ufual way. This method is now adopted for joining together the conducting pipes of the machines for extinguishing fires, an operation which was extremely troublefome before this improvement. The conduit pipe Eefe(fig. 23.) is faftened to the bottom of the barrel, and the discharging pipe gb to its top, in the fame manner. But to return to the gage; the bent pipe cba enters the box st near one fide, and obliquely, and the gage pipe q is inferted through its bottom towards the oppofite fide. The use of this box is to catch any drops of mercury which may fometimes be dafhed up through the gage pipe by an accidental ofcillation. This, by going through the paffages of the pump, would corrode them, and would act particularly on the joints, which are generally foldered with tin. When this happens to an air-pump, it must be cleaned with the most scrupulous attention, otherwise it will be quickly destroyed. 133 This account of Smeaton's pump is fufficient for Great enabling the reader to understand its operation and to powers of fee its fuperiority. It is reckoned a very fine pump of this pump. the ordinary construction which will rarefy 200 times, or raife the gage to 29.85, the barometer ftanding at 30. But Mr Smeaton found, that his pump, even after long wing, raised it to 29.95, which we confider as equivalent to rarefying 600 times. When in fine order, he found no bounds to its rarefaction, frequently raising the gage as high as the barometer; and he thought its performance fo perfect, that the barometer-gage was not fufficiently delicate for meafuring the rarefaction. He therefore fubftituted the fyphon gage already defcribed, which he gives fome reafons for preferring ; but even this he found not fufficiently fenfible. He contrived another, which could be carried to 3 any 136 Another Plate COCCIL. Air pump any degree of fenfibility. It confifted of a glafs body A (fig. 24.), of a pear shape, and was therefore called the pear-gage. This had a fmall projecting orifice at trivance B, and at the other end a tube CD, whofe capacity was of Smea- the hundredth part of the capacity of the whole veffel. to's This was fufpended at the flip-wire of the receiver, and there was fet below it a fmall cup with mercury. When the pump was worked, the air in the pear-gage was rarefied along with the reft. When the rarefaction was brought to the degree intended, the gage was let down till B reached the bottom of the mercury. The external air being now let in, the mercury was raised into the pear, and flood at fome height E in the tube CD. The length of this tube being divided into 100 parts, DE and those numbered from D, it is evident that will DB exprefs the degree of rarefaction which had been produced when the gage was immerfed into the mercury: or if DC be of the whole capacity, and be divided into 100 parts by a fcale annexed to it, cach unit of the fcale will be rs of the whole. 137 Very inge. 138 It excited tion of the literary world. This was a very ingenious contrivance, and has been the means of making fome very curious and important discoveries which at prefent engage the attention of philofophers. By this gage Mr Smeaton found, that his pump frequently rarefied a thousand, ten thoufand, nay an hundred thoufand, times. But though he in every inftance faw the great fuperiority of his pump above all others, he frequently found irregularities which he could not explain, and a want of correfpondence between the pear and the barometer gages which puzzled him. The pear-gage frequently indicated a prodigious rarefaction, when the barometer-gage would not how more than 600. Thefe unaccountable phenomena excited the curiofity the atten of philofophers, who by this time were making continual ufe of the air-pump in their meteorological refearches, and much interested in every thing connected with the ftate or conftitution of elastic fluids. Mr Nairne, a moft ingenious and accurate maker of philofophical inftruments, made many curious experiments in the examination and comparison of Mr Smeaton's pump with thofe of the ufual conftruction, attending to every circumstance which could contribute to the inferiority of the common pumps or to their improvement, fo as to bring them nearer to this rival machine. This rigorous comparison brought into view feveral circumftances in the conftitution of the atmospheric air, and its relation to other bodies, which are of the moft extenfive and important influence in the operations of nature. We fhall notice at prefent fuch only as have a relation to the operation of the air-pump in extracting AIR from the it by Mr Nairne receiver. 139. Exper Mr Nairne found, that when a little water, or even a ments with bit of paper damped with water, was expofed under the receiver of Mr Smeaton's air-pump, when in the moft perfect condition, raising the mercury in the barometergage to 29.95, he could not make it rife above 29.8 if Fahrenheit's thermometer indicated the temperature 47°, nor above 29.7 if the thermometer flood at 550; and that to bring the gage to this height and keep it there, the operation of the pump muft be continued for a long time after the water had disappeared or the paper become perfectly dry. He found that a drop of Spirits, or paper moiftened with fpirits, could not in thofe circrmftances allow the mercury in the gage to Air-pump. rife to near that height; and that fimilar effects followed from admitting any volatile body whatever into the receiver or any part of the apparatus. 140 This fhowed him at once how improper the direc- Show the tions were which had been given by Gueriche, Boyle, improprieGravefande, and others, for fitting up the air-pump for Y of foaking the leas experiment, by foaking the leather in water, covering ther with the joints with water, or in fhort, admitting, water or water, any other volatile body near it. 141 He therefore took his pumps to pieces, cleared them And the of all the moisture which he could drive from them by utility of ufing olive heat, and then leathered them anew with leather foaked oil and in a mixture of olive oil and tallow, from which he had tallow. expelled all the water it ufually contains, by boiling it till the first frothing was over. When the pumps were fitted up in this manner, he uniformly found that Mr Smeaton's pump rarefied the gage to 29.95, and the beft common pump to 29.87, the first of which he computed to indicate a rarefaction to 600, and the other to 230. But in this ftate he again found that a piece of damp paper, leather, wood, &c. in the receiver, reduced. the performance in the fame manner as before. 142 But the most remarkable phenomenon was, that when A remark. he made ufe of the pear-gage with the pump cleared from able pheno all moisture, it indicated the fame degree of rarefaction menon with the barometer-gage: but when he expofed a bit of paper moiftened with fpirits, and thus reduced the rarefaction of the pump to what he called 50, the baro. meter-gage ftanding at 29.4, the pear-gage indicated a rarefaction exceeding 100,000; in fhort, it was not measurable; and this phenomenon was almost constant. Whenever he expofed any fubftance fufceptible of evaporation, he found the rarefaction indicated by the barometer-gage greatly reduced, while that indicated by the pear-gage was prodigioufly increased; and both thefe effects were more remarkable as the subject was of easier evaporation, or the temperament of the air of the chamber was warmer. 143 This uniform refult fuggefted the true caufe. Water Accounted boils at the temperature 212, that is, it then con-for. werted into a vapour which is permanently elastic while of that temperature, and its elafticity balances the preffure of the atmosphere. If this preffure be diminished by rarefying the air above it, a lower temperature will now allow it to be converted into elaftic vapour, and keep it in that state. Water will boil in the receiver of an air-pump at the temperament 96, or even under it. Philofophers did not think of examining the state of the vapour in temperatures lower than what produced ebullition. But it now appears, that in much lower heats than this the fuperficial water is converted into elaftic vapour, which continues to exhale from it as long as the water lafts, and, fupplying the place of air in the receiver, exerts the fame elasticity, and hinders the mercury from rifing in the gage in the fame manner as fo much air of equal elafticity would have done. this ac When Mr Nairne was exhibiting these experiments Experi to the Honourable Henry Cavendish in 1776, this gen-ments iltleman informed him that it appeared from a series of luftrating experiments of his father Lord Charles Cavendish, that when water is of the temperature 72, it is converted into vapour, under any preffure lefs than three-fourths of an inch of mercury, and at 41° it becomes vapour when the preffure is lefs than one-fourth of an inch: coust. 146 meter Air-pump. Even mercury evaporates in this manner when all preffure Let us now confider what must happen in the pear- Air-pump is removed. A dewy appearance is frequently obferved gage. As the air and vapour are continually drawn off covering the infide of the tube of a harometer, where from the receiver, the air in the pear expands and goes Confequenwe ufually fuppofe a vacuum. This dew, when viewed off with it. We shall fuppofe that the generated va-ces of this through a microscope, appears to be a fet of detached pour hinders the gage from rifing beyond 29.5. Du-different in globules of mercury, and upon inclining the tube fo ring the continued werking of the pump, the air in the pear that the mercury may afcend along it, thefe globules the pear, whofe elafticity is 0.5, flowly mixes with and baro. will be all licked up, and the tube become clear. The the vapour at the mouth of the pear, and the mix- -gages. dew which lined it was the vapour of the mercury con- ture even advances into its infide, fo that if the densed by the fide of the tube; and it is never obferved pumping be long enough continued, what is in the but when one fide is exposed to a stream of cold air pear is nearly of the fame compofition with what is from a window, &c. in the receiver, confifting perhaps of 20 parts of vapour and one part of air, all of the elasticity of 0.5. When the pear is plunged into the mercury, and the external air allowed to get into the receiver, the mercu ry rifes in the pear-gage, and leaves not but 60 60X20 or of it filled with common air, the vapour having collapfed into an invifible atom of water. Thus the pear-gage will indicate a rarefaction of 1200, while the elafticity of the included fubftance diminished 60 the barometer-gage only fhowed 60, that is, fhowed times. The conclufion to be drawn from these two 145 Air and mixed to To return to the vapour in the air-pump receiver, it must be observed, that as long as the water continues to yield it, we may continue to work the pump; and it will be continually abstracted by the barrels, and difcharged in the form of water, because it collapfes as foon as expofed to the external preffure. All this while the gage will not indicate any more rarefaction, because the thing immediately indicated by the baro meter-gage is diminished elaflicity, which does not happen here. When all the water which the temperature of the room can keep elastic has evaporated under a certain preffure, suppose an inch of mercury, the gage ftanding at 29.5, the vapour which now fills the receiver expands, and by its diminished elafticity the gage rifes, and now fome more water which had been attached to bodies by chemical or corpufcular attraction is detached, and a new fupply continues to fupport the gage at a greater height; and this goes on continually till almost all has been abstracted: but there will remain fome which no art take can' away; for as it paffes through the barrels, and gets between the pifton and the top, it fucceffively collapfes into water during the afcent of the piston, and again expands into vapour when we push the pifton down again. Whenever this happens there is an end of the rarefaction. While this operation is going on, the air comes out vapour nos along with the vapour; but we cannot fay in what prouniformly portion. If it were always uniformly mixed with the gether. vapour, it would diminish rapidly; but this does not appear to be the cafe. There is a certain period of rarefaction in which a tranfient cloudinefs is perceived in the receiver. This is watery vapour formed at that degree of rarefaction, mingled with, but not diffolved in or united with, the air, otherwise it would be tranfparent. A fimilar cloud will appear if damp air be ad mitted fuddenly into an exhausted receiver. The va pour, which formed an uniform transparent mass with the air, is either fuddenly expanded and thus detached from the other ingredient, or is fuddenly let go by the air, which expands more than it does. We cannot affirm with probability which of thefe is the cafe: different compofitions of air, that is, air loaded with vapours from different fubstances, exhibit remarkable differen ces in this respect. But we fee from this and other phenomena, which shall be mentioned in their proper places, that the air and vapour are not always intimately united; and therefore will not always be drawn out to gether by the air-pump. But let them be ever fo confufedly blended, we fee that the air must come out along with the vapour, and its quantity remaining in the receiver must be prodigiously diminished by this affocia tion, probably much more than could be, had the receiver only contained pure air. I 1200 I I meafures (the one of the rarefaction of air, and the as 60 147 account The only obfcure part of this account is what relates Difficulty to the compofition of the matter which filled the pear-in acco gage before the admiffion of the mercury. It is not eafy fome of to fee how the vapour of the receiver comes in by a nar-thefe conrow mouth while the air is coming out by the fame paffage. fequences. Accordingly it requires a very long time to produce this extreme rarefaction in the pear-gage; and there are great irregularities in any two fuccceding experiments, may be feen by looking at Mr Nairne's account of them in Philofophical Tranfactions, Vol. LXVII. Some vapours appear to have mixed much more readily with the air than others; and there are some unaccountable cafes where vitriolic acid and fulphureous bodies were included, in which the diminution of denfity indicated by the pear-gage was uniformly lefs than the diminution of elafticity indicated by the barometer-gage. It is enough for us at present to have established, by unquestionable facts, this production of elastic vapour, and the neceffity of attending to it, both in the construction of the air-pump and in drawing refults from experiments exhibited in it. 148 excites to new im Mr Smeaton's pump, when in good order, and per-The fupefectly free from all moisture, will in dry weather rarefy riority air about 600 times, raifing the barometer-gage to with- this pump in of an inch of a fine barometer. This was a performance fo much fuperior to that of all others, and by prove means of Mr Nairne's experiments opened fo new a field ments. of obfervation, that the air pump once more became a capital inftrument among the experimental philofophers. The caufes of its fuperiority were alfo fo diftinct, that artifts were immediately excited to a farther improvement of the machine; fo that this becomes a new epoch in its history. This is one imperfection which Mr Smeaton has not attempted to remove. The difcharging valve is ftill open ed this pump not find that this improvement has been adopted so as attempted to become general. Indeed the quantity of air which remains in the paffage to this valve is fo exceedingly little, that it does not feem to merit attention. Suppofing the valve-hole of an inch wide and as deep (and it need not be more), it will not occupy more than part of a barrel twelve inches long and two inches wide. 150 131 Mr Smeaton, by his ingenious conftruction, has great ly diminished, but has not annihilated, the obstructions to the paffage of the air from the receiver into the barrel. His fuccefs encouraged farther attempts. One of the first and most ingenious was that of Profeffor Ruffel of the university of Edinburgh, who about the year 1770 constructed a pump in which both cocks and valves .were avoided. The pifton is folid, as reprefented in fig. 25. and its By Ruffel, rod paffes through a collar of leather on the top of the barrel. This collar is divided into three portions by two brafs rings a, b, which leave a very small space round the pifton rod. The upper ring a communicates by means of a lateral perforation with the bent tube Imn, which enters the barrel at its middle n. The lower ring t communicates with the bent tube cd, which communicates with the horizontal paffage de, going to the middle of the pump plate. By the way, however, it communicates alfo with a barometer gage po, ftand. ing in a ciftern of mercury o, and covered with a glafs tube close at the top. Beyond e, on the oppofite circumference of the receiver plate, there is a cock or plug f communicating with the atmosphere. The pifton rod is clofely embraced by the three collars of leather; but, as already faid, has a free fpace round it in the two brafs rings. To produce this pref. fure of the leathers to the rod, the brafs rings which feparate them are turned thinner on the inner fide, fo that their crofs fection along a diameter would be a taper wedge. In the fide of the pifton rod are two cavities qr, ts, about one-tenth of an inch wide and deep, and of a length equal to the thickness of the two rings a, b, and the intermediate collar of leathers. Thefe cavities are fo placed on the piflon-rod, that when the piston is applied to the bottom of the barrel, the cavity in the upper end of the rod has its upper end oppofite to the ring a, and its lower end oppofite to the ring b, or to the mouth of the pipe cd. Therefore, if there be a void in the barrel, the air from the receiver will come from the pipe c d into the cavity in the pifton rod, and by it will get paft the collar of leather between the rings, and thus will get into the fmall interftice between the rod and the upper ring, and then into the pipe /mn, and into the empty barrel. When the pifton is drawn up, the folid rod immediately fhuts up this paffage, and the pifton drives the air through the difcharging valve k. When it has reached the top of the barrel, and is closely applied to it, the cavity qr is in the fituation in which is formerly was, and the communication is again opened between the receiver and the empty barrel, and the air is again diffused between them. Pufhing down the pifton expels the air by the lower difcharging pipe and walve bi; and thus the operation may be continued. VOL. XV. Part I. 105 This must be acknowledged to be a moft fimple and Air-pump. ingenious conftruction, and can neither be called a cock nor a valve. It feems to oppose no obstruction whatever: and it has the fuperior advantage of rarefying both during the afcent and the defcent of the pifton, doubling the expedition of the performance, and the operator is not oppofed by the preffure of the atmofphere except towards the end of each ftroke. The expedition, however, is not fo great as one fhould expect; for nothing is going on while the pifton is in motion, and the operator muft ftop a while at the end of each ftroke, that the air may have time to come through this long, narrow, and crooked paffage, to fill the barrel. But the chief difficulty which occurred in the execution arose from the clammy oil with which it was neceffary to impregnate the collar of leathers. These were always in a state of ftrong compreffion, that they might clofely grafp the pifton rod, and prevent all paffage of air during the motion of the pifton. Whenever therefore the cavities in the pifton rod come into the fituations neceffary for connecting the receiver and barrel, this oil is fqueezed into them, and choaks them up. Hence it always happened that it was fome time after the stroke before the air could force its way round the piston rod, carrying with it the clammy oil which choaked up the tube Imn; and when the rarefaction had proceeded a certain length, the diminished elafticity of the air was not able to make its way through these obstructions. The death of the ingenious author put a stop to the improvements by which he hoped to remedy this defect, and we have not heard that any other person has fince attempted it. We have inferted it here, because its principle of conftruction is not only very ingenious, but entirely different from all others, and may furnish very ufeful hints to those who are much engaged in the conftruction of pneumatic engines. 152 In the 734 volume of the Philofophical Tranfactions, By Haas Mr Tiberius Cavallo has given the defcription of an and Hur. air-pump contrived and executed by Meffrs Haas and ter, Hurter, inftrument-makers in London, where these artifts have revived Guericke's method of opening the barrel-valve during the laft ftrokes of the pump by a force acting from without. We fhall infert fo much of this defcription as relates to this diftinguishing circumftance of its conftruction. Fig. 26. reprefents a fection of the bottom of the barrel, where AA is the barrel and BB the bottom, which has in its middle a hollow cylinder CCFF, projecting about half an inch into the barrel at CC, and extending a good way downwards to FF. The fpace between this projection and the fides of the barrel is filled up by a brafs ring DD, over the top of which is ftrained a piece of oiled filk EE, which performs the office of a valve, covering the hole CC. But this hole is filled up by a piece of brafs, or rather an affemblage of pieces fcrewed together GGHHII. It confifts of three projecting fillets or fhoulders GG, HH, II, which form two hollows between them, and which are filled with rings of oiled leather OO, PP, firmly fcrewed together. The extreme fillets GG, II, are of equal diameter with the infide of the cylinder, fo as to fill it exactly, and the whole ftuffed with oiled leather, flide up and down without allowing any air to pafs. The mid le fillet HH is not fo broad, but thicker. In the upper fillet GG.there is formed a fhallow dish about of an inch deep it exactly fills, it carries up the air before it, and ex- Air-pomp Air-pump. deep and wide. This dish is covered with a thin plate, pierced with a grating like Mr Smeaton's valve-plate. There is a perforation VX along the axis of this piece, which has a paffage out at one fide H, through the middle fillet. Oppofite to this paffage, and in the fide of the cylinder CCFF, is a hole M, communicating with the conduit pipe MN, which leads to the receiver. Into the lower end of the perforation is screwed the pin KL, whose tail L paffes through the cap FF. The tail L is connected with a lever RQ, moveable round the joint Q. This lever is pushed upwards by a spring, and thus the whole piece which we have been defcribing is kept in contact with the flip of oiled filk or valve EE. This is the ufual fituation of things. 353 154 By Prince, Plate Ccccn. Now fuppofe a void formed in the barrel by drawing up the pifton; the elafticity of the air in the receiver, in the pipe NM, and in the paffage XV, will prefs on the great furface of the valve exposed through the grating, will raise it, and the pump will perform precifely as Mr Smeaton's does. But fuppofe the rarefaction to have been fo long continued, that the air is no longer able to raise the valve; this will be seen by the mercury rifing no more in the pump-gage. When this is perceived, the operator must prefs with his foot on the end R of the lever RQ. This draws down the pin KL, and with it the whole hollow plug with its grated top. And thus, instead of raifing the valve from its plate, the plate is here drawn down from the valve. The air now gets in without any obftruction whatever, and the rarefaction proceeds as long as the piston rifes. When it is at the top of the barrel, the operator takes his foot from the lever, and the spring preffes up the plug again and fhuts the valve. The piston rod paffes through a collar of leather, as in Mr Smeaton's pump, and the air is finally discharged through an outward valve in the top of the barrel. These parts have nothing peculiar in them. This is an ingenious contrivance, fimilar to what was adapted by Guericke himself; and we have no doubt of thefe pumps performing extremely well if carefully made and it feems not difficult to keep the plug perfectly air-tight by fupplying plenty of oil to the leathers. We cannot fay, however, with precifion what may be expected from it, as no account has been given of its effects befides what Mr Cavallo published in Philofophical Tranfactions 1783, where he only fays, that when it had been long used, it had, in the course of fome experiments, rarefied 600 times. No diftinct account has as yet been given of the performance of this pump. We only learn that great in conveniences were experienced from the ofcillations of the mercury in the gage. As foon as the piston comes into the ciftern, the air from the receiver immediately rushes into the barrel, and the mercury fhoots up in the gage, and gets into a state of ofcillation. The fubfequent rife of the pifton will frequently keep time with the fecond ofcillation, and increafe it. The defcent of the pifton produces a downward ofcillation, by allowing the air below it to collapfe; and, by improperly timing the ftrokes, this ofcillation becomes fo great as to make the mercury enter the pump. To prevent this, and a greater irregularity of working as a condenfer, valves were put in the piston: but as these require force to open them, the addition feemed rather to increafe the evil, by rendering the ofcillations more fimul taneous with the ordinary rate of working. If this could be got over, the construction feems very promifing. It appears, however, of very difficult execution. It has many long, flender, and crooked paffages, which must be drilled through broad plates of brafs, fome of them appearing fcarcely practicable. It is rare to find plates and other pieces of brafs without air-holes, which it would be very difficult to find out and to close; and it muft be very difficult to clear it of obftructions: so that it appears rather a fuggeftion of theory than a thing warranted by its actual performance. Mr Lavoifier, or fome of the naturalifts who were 155 It confifts of two barrels l, m, fig. 28. with folid pi- |