Laurie, land-furveyor in Edinburgh, about 25 years ago, and improved fince his death.

ABCD (fig. 76, pl. 282.) is an iron cylinder, truly bored within, and evafated a-top like a cup. EFGH is another, truly turned both without and within, fomewhat lefs than the inner diameter of the first cylinder. This cylinder is close above, and hangs from the end of a lever moved by a machine. It is alfo loaded with weights at N. KILM is a third cylinder, whofe outfide diameter is fomewhat lefs than the infide diameter of the fecond. This inner cylinder is fixed to the fame bottom with the outer cylinder. The middle cylinder is loofe, and can move up and down between the outer and inner cylinders without rub. bing on either of them. The inner cylinder is perforated from top to bottom by three pipes OQ, SV, PR. The pipes OQ, PR have valves at their upper ends O, P, and communicate with the external air below. The pipe SV has a hori. zontal part VW, which again turns upwards, and has a valve at top X. This upright part WX is in the middle of a ciftern of water fhkg. Into this ciftern is fixed an air-chest aŸZb, open below, and having at top a pipe e de terminating in the tuyere at the furnace. When the machine is at rest, the valves X, O, P, are shut by their own weights, and the air-cheft is full of water. In this state, the middle cylinder EFGH is drawn up by the machinery till its lower brims F and G are equal with the top RM of the inner cylinder. Now pour in water or oil between the outer and middle cylinders: it will run down and fill the space between the outer and inner cylinders. Let it come to the top of the inner cylinder.

C

Now let the loaded middle cylinder defcend. It cannot do this without compreffing the air which is between its top and the top of the inner cylinder. This air being compreffed will caufe the water to defcend between the inner and middle cylinders, and rife between the middle and outer cylinders fpreading into the cup; and as the middle cylinder advances downwards, the water will defcend farther within it and rise farther without it. When it has got fo far down, and the air has been fo much compreffed, that the difference between the furface of the water on the infide and outfide of this cylinder is greater than the depth of water between X and the furface of the water fg, air will go out by the pipe SVW, and will lodge in the air-cheft, and will remain there if c be hut, which we fhall fuppofe for the prefent. Pushing down the middle cylinder till the partition touch the top of the inner cylinder, all the air which was formerly between them will be forced into the air-chest, and will drive out water from it. Draw up the middle cylinder, and the external air will open the valves O,P, and again fill the space between the middle and inner cylin. ders: for the valve X will fhut, and prevent the regrefs of the condenfed air. By pushing down the middle cylinder a fecond time, more air will be forced into the air-cheft, and it will at last escape by getting out between its brims Y, Z and the bottom of the ciftern; or if we open the paffage, it will pass along the conduit c d e to the tuyere, and form a blast.

The operation of this machine is fimilar to M.

Haskin's quickfilver pump defcribed by Defaguliers at the end of the 2d vol. of his Exper. Philof The force which condenses the air is the load on the middle cylinder. The ufe of the water between the inner and outer cylinders is to prevent this air from efcaping; and the inner cylinder thus performs the office of a piston, having no friction. It is neceflary that the length of the outer and middle cylinders be greater than the depth of the regulator ciftern, that there may be a fufficient height for the water to rife between the middle and outer cylinders, to balance the compressed air, and oblige it to go into the air-cheft. A large blast-furnace will require the regulator ciftern g feet deep, and the cylinders about 6 or 7 feet long. It is in fact a pump without friction, and is perfectly air-tight. The quickness of its operation depends on the small space between the middle cylinder and the two others; and this is the only use of these two. Without thefe it would be fimilar to the engine at Chatillon, and operate more unequally and flowly. Its only imperfection, is, that if the cylinder begin its motion of ascent or descent rapidly, as it will do when worked by a team-engine, there will be fome danger of water dafhing over the top of the inner cylinder and getting into the pipe SV; but should this happen, an iffue can easily be contrived for it at V, covered with a loaded valve v. This will never happen if the cylinder is moved by a crank. One blowing cylinder only is represented here, but two may be used.

This form of bellows is the most perfect of any, and fit for all ufes where ftanding bellows are required. They will be cheaper than any other fort for common purposes. For a common fmith's forge they may be made with fquare wooden boxes inftead of cylinders. They are alfo eafily repaired. They are perfectly tight; and they may be made with a blast almost perfectly uniform, by making the ciftern in which the air-cheft flands of confiderable dimensions. When this is the cafe, the height of water which regulates the blast, will vary very little.

The leading parts of the conftruction of blaft machines have been defcribed as far only as was neceffary for understanding their operation, and enabling an engineer to erect them in the most commodious manner. Views of complete machines would not have added to our reader's information. That their parts may be fo propostioned that they fhall perform what is expected from them, the engineer fhould know what fize of bellows, and what load on the board, or pifton, and what fize of tuyere will give the blatt which the fervice requires, and what force must be employed to give them the neceflary degree of motion. For these purposes we must confider the efflux of the compreffed air through the tuyere.

That we may proportion every thing to the power employed, we must recollect, that if the pifton of a cylinder employed for expelling air be preffed down with any force p, it must be confidered as fuperadded to the atmosphoric pressure P on the fame pifton, in order that we may compare the velocity of efflux with the known velocity V with which the air rufhes into a void. By what has been formerly delivered, it appears

that this velocity v=V ×,

PXP

where P is the preffure of the atmosphere on the pifton, and p the additional load laid on it. This velocity is expreffed in feet per fecond; and, when multiplied by the area of the orifice (alfo exprefsed in fquare feet), it will give us the cubical feet of condensed air expelled in a fecond; but the bellows are always to be filled again with common air, and therefore we want to know the quantity of common air which will be expelled, for it is this which determines the number of ftrokes which must be made in a minute, in order that the proper supply may be obtained. Therefore recollect that the quantity expelled from a given orifice with a given velocity, is in the proportion of the denfity; and that when D is the denfity of common air produced by the preffure P, the denfity d produced by the preffure P+

is Dx; or if D be made 1, we have d

P+p.

=P

Therefore, calling the area of the orifice expreffed in fquare feet O, and the quantity of common air, or the cubic feet expelled in a fecond

Q, we have Q=VX0X

It will be fufficiently exact for all practical purpoles to fuppofe P to be 15 pounds on every fquare inch of the pifton; and p is then conveniently expreffed by the pounds of additional load on every fquare inch; we may alfo take V=1332 feet. As the orifice through which the air is expelled is generally very fmall, never exceeding 3 inches in diameter, it will be more convenient to exprefs it in fquare inches; which being the one 144th of a fquare foot, we fhali have the cubic feet of common air expelled in a second or Q= 1 P+P = 0X9'25 X p 144 O P+pXP x2+

1332

P+ and this seems to be as fimple an expreffion as we can obtain.

This may be illuftrated by an example in numbers. Let the area of the piston be four square feet, and the area of the round hole through which the air is expelled be two inches, its diameter being r6, and let the load on the pifton be 1728 pounds: this is three pounds on every fquare inch. We have P=15, p=3, P Xp=18, and O=2; 18,

therefore we will have Q=2 × 9°25 X

15

=9053 cubic feet of common air expelled in a fecond. This will however be diminished at leaft

then defcend uniformly at the above rate, expelling 6 cubic feet of common air in a fecond.

one third by the contraction of the jet ; and therefore the fupply will not exceed fix cubic feet per fecond. Suppofing, therefore, that this blowing machine is a cylinder or prifm of this dimenfion in its fection, the pifton fo loaded would (after having compreffed the air) defcend about 15 inches in a fecond: It would firft fink one sth of the whole length of the cylinder pretty fuddenly, till had reduced the air to the denfity, and would

The computation is made much in the fame way for bellows of the common form, with this additional circumftance, that as the loaded board moves round a hinge at one end, the preffure of the load muft be calculated accordingly. The computation, however, becomes a little intricate, when the form of the loaded board is not rectangular: it is almost useless when the bellows have flexible fides, either like fmith's bellows or like organ bellows, because the change of figure during their motion makes continual variation on the compreffing powers. It is therefore chiefly with refpect to the great wooden bellows, of which the upper board flides down between the fides, that the above calculation is of fervice.

The propriety, however, of this piece of information is evident: we do not know precisely the quantity of air neceffary for animating a furnace; employed for expeiling the air that may be thought but this calculation tells us what force must be neceffary. If we have fixed on the strength of the blaft, and the diameter of the cylinder, we learn the weight with which the piston must be loaded; the length of the cylinder determines its capacity, the above calculation tells the expense per fecond; hence we have the time of the pifton's coming to the bottom. This gives us the number of ftrokes per minute: the load must be lifted up by the machine this number of times, making the time of afcent precifely equal to that of defcent; otherwife the machine will either catch and stop the defcent of the pifton, or allow it to lie inactive for a while of each ftroke. Thefe circumftances determine the labour to be performed by the machine, and it must be conftructed accordingly. Thus the engineer will not be affronted by its failure, nor will he expend needlefs power and coft.

In machines which force the pifton or bellowsboard with a certain determined motion, different from what arifes from their own weight, the computation is extremely intricate. When a pifton moves by a crank, its motion at the beginning and end of each stroke is flow, and the compreffion and efflux is continually changing: we force required. Every time the pifton is drawn can however approximate to a statement of the up, a certain space of the cylinder is filled again pelled during the defcent of the pifton. A cerwith air of the common denfity; and this is extain number of cubic feet of common air is therefore expelled with a velocity which perhaps continually varies; but there is a medium velocity with which it might have been uniformly expelled, and a preffure corresponding to this velocithe area of the blaft-hole (or rather by this area ty. To find this, divide the area of the pifton by multiplied by 0613, in order to take the effect of the ftroke performed in a fecond by the quotient the contracted jet), and multiply the length of arifing from this divifion; the product is the medium velocity of the air (of the natural density). Then find by calculation the height through which a heavy body muft fall in order to acquire this velocity; this is the height of a column of homogenous air which would expel it with this velocity. The weight of this column is the leaft

F&

force

force that can be exerted by the engine: but this force is too small to overcome the resistance in the middle of the ftroke, and it is too great even for the end of the ftroke, and much too great for the beginning of it. But if the machine is turned by A very heavy water-wheel, this will act as a regujator, accummulating in itfelf the fuperfluous force during the two favourable pofitions of the crank, and exerting it by its vis infita during the time of greateft effort. A force not greatly exceeding the weight of this column of air will therefore fuffice. On the other hand, if the ftrength of the blaft be determined, which is the general state of the prob. lem, this determines the degree of condenfation of the air, and the load on the fquare inch of the pifton, or the mean force which the machine muft excrt on it. A table which will be given prefentdy, determines the cubic feet of common air expelled in a fecond, correfponding to this load. This combined with the propofed dimensions of the cylinder, will give the defcent of the pifton or the length of the ftroke.

Thefe general obfervations apply to all forms of bellows; and without a knowledge of them no perfon can erect a machine for working them without total uncertainty or fervile imitation. In order, therefore, that they may be useful to fuch as are not accustomed to the management of even thefe fimple formula, we infert the following fhort table of the velocity and quantity of air difcharged from a cylinder whofe pifton is loaded with the pounds contained in the firft column on every fquare inch. The 2d column contains the veloci. ty with which the condensed air ruthes out through any small hole; and the 3d column is the cubic feet difcharged from a hole whofe area is a fquare inch; column 4th contains the mean velocity of air of the common density; and column 5th is the cubic feet of common air difcharged; the 6th.co. lumn is the height in inches at which the force of the blaft would fupport a column of water if a pipe were inferted into the fide of the cylinder. This is an extremely proper addition to fuch machines, fhowing at all times the power of the machines, and teaching us what intensity of blaft is employed for different purposes. The table is computed from the fuppofition that the ordinary preffure of the air is 15 pounds on a fquare inch. This is fomewhat too great, and the efore the velocities are a little too fmall; but the quantities difcharged will be found about too great (with out affecting the velocities) on account of the conVergency of the stream.

This table extends far beyond the limits of dinary use, very few blast-furnaces having a f exceeding 60 inches of water.

We shall conclude this account of blowing chines with a description of a small one for a bi pipe. Fig. 77. Pl. 282. ABCD, is a veffel cont ing water, about two feet deep. EFGH is the box of the blower, open below, and having a ILK rifing up from it to a convenient height; arm ON which grafps this pipe carries the la N: the blow-pipe LM comes from the top of upright pipe. PKQ is the feeding pipe reach near to the bottom of the veffel. Water b poured into the veffel below, and its cover b put on, which fits the upright pipe, and tou two stands a, a, projecting from it, blow in a qu tity of air by the feeding pipe PQ; this expels water from the air-box, and occafions a pref which produces the blast through the blow-pipe

The ancients had a hydraulic machine ca Hiero's or Hero's fountain, from the Syracufan invented it. As it has in modern times been verted into a moft powerful engine for railing ter from the bottom of mines, we fhall give a fcription of it, both in its ancient and modern f In its original ftate it confifted of two ver KLMN, (Plate 283, fig. 80.) and OPQR, wh are clofe on all fides. A tube AB, having a nel a-top, paffed through the uppermoft vo without communicating with it, being folde into its top and bottom. It alfo paffed thro the top of the under veifel, where it was alfo dered, and reached almoft to its bottom. tube was open at both ends. There was ano open tube ST, soldered into the top of the un veffel and the bottom of the upper vcffel, re. ing almost to its top. Thefe two tubes ferved fupport the upper veffel. A third tube GF foldered into the top of the upper veffel, reached almoft to its bottom. This tube open at both ends, but the orifice G very im Suppofe the uppermoft veffel filled with wate the height EN, Ee being its furface a little be T.

Π

Stop the orifice G with the finger, and p in water at A. This will defcend through and comprefs the air in OQRP into lefs re Suppofe the water in the under veffel to have quired the furface Cc, the air which formerly cupied the whole of the spaces OFQR and K E will now be contained in the spaces o Pe C KL E; and its elafticity will be in equili with the weight of the column of water, w! bate is the furface E e, and whofe height is As this preffure is exerted in every point of the it will be exerted on the furface Ee of the w of the upper veffel; and if the pipe FG were tinued upwards, the water would be fuppo in it to a height e H above E e, equal to Therefore, if the finger be now taken from off orifice G, the water will spout up to the f height as if it had been immediately forced out a column of water A c without the interventio the air, that is, nearly to H. If, instead of the nel at A, the veffel have a brim which will ca the water difcharged at G to run down the AB, this fountain will play till all the wate the upper veffel is expended. The opera of the fecond fountain will be better underst f

and the performance is 32 raised 96, and they are in the proportion of 66 X 136 to 32 X 96, or of I to o'3422, or nearly as 3 to 1. This is fuperior to the performance of the moft perfect undershot mill, even when all friction and irregular obftructions are neglected; and is not much inferior to any overfhot pump-mill that has yet been erected. Then it must be confidered how inferior in original expense this fimple machine muft be to a mill of any kind which would raise 10 cubic feet 96 feet high in a minute, and how small the repairs on it need be, when compared with a mill. And, laftly, fuch a machine can be used where no mill whatever can be put in motion. A fmall ftream of water, which would not move any kind of wheel, will here raife of its own quantity to the fame height; working as faft as it is fupplied.

On all thefe accounts, the Hungarian machine eminently deserves the attention of mathematicians and engineers, to bring it to its utmoft per fection, and into general ufe. There are fituations where this machine may be very useful. Thus, where the tide rifes 17 feet, it may be used for compreffing air to of its bulk; and a pipe leading from a very large veffel inverted in it, may be used for raifing the water from a vessel of of its capacity 17 feet high; or if this veffel has only one 16th of the capacity of the large one fet in the tide-way, two pipes may be led from it; one into the small veffel, and the other into an equal veffel 16 feet higher, which receives the water from the firft. Thus one 16th of the water may be raised 34 feet, and a smaller quantity to a ftill greater height; and this with a kind of power that can hardly be applied in any other way. Machines of this kind are defcribed by Schottus, Sturmius, Leupold, and other old writers; and opportunities may offer of making them highly ufeful. A gentleman's house in the country may thus be fupplied with water by a machine that will coft little, and hardly go out of repair.

The laft pneumatical engine which we shall de fcribe, is the common fanners used for winnow ing grain, and for drawing air out of a room The wings of the fanners are inclofed in a cylin der or drum, whose circular fides have a larg opening BDE (fig. 79, Plate 283.) round the centre to admit the air. By turning the wings rapidly round, the air is hurried round along with them and thus acquires a centrifugal tendency, by which it preffes ftrongly on the outer rim of the drum: this is gradually detached from the circle as at KI, and terminated in a trunk IHGF, which goes off in a tangential direction; the air therefore is driven along this paffage.

If the wings were difpofed in planes paffing through the axis C, the compreffion of the air by their anterior surface would give it fome tendency to escape in every direction, and would obftruct in fome degree the arrival of more air through the fide-holes. They are therefore reclined a little backward, as reprefented in the figure. It may be shown that their best form would be that of a hyperbolic spiral abc; but the ftraight form approaches fufficiently near to the most perfect fhape. Much labour is loft, however, in carrying the air round those parts of the drum where it cannot efcape. The fanners would either draw or discharge almost twice as much air if an opening were made all round one fide. This could be gradually contracted (where required for winnowing) by a furrounding cone, and thus directed against the falling grain: this has been verified by actual trial. When used for drawing air out of a room for ventilation, it would be much better to remove the outer fide of the drum entirely, and let the air fly freely off on all fides; but the flat fides are neceffary, in order to prevent the air from arriving at the fanners any other way but through the central holes, to which trunks fhould be fitted leading to the apartment which is to be ventilated.

PNE

PNEUMATOCELE, a rupture occafioned by wind: a tumour of the fcrotum diftended by wind. Afh.

PNEUMATO-CHEMICAL APPARATUS. See CHEMISTRY, Index.

PNEUMATODES, fhortnefs of breath. PNEUMATOLOGIST, n.. one skilled in pneumatology; a writer on pneumatology. (1.)* PNEUMATOLOGY. n. f. [πvivarohoyia.] The doctrine of spiritual existence.

(2.) PNEUMATOLOGY. See ANGEL, APPARITION, DEMON, DEMONIAC, DEVIL, GENIUS, GHOST, METAPHYSICS, 5, and Sect. xxxv, XXXVI; PNEUMATICS, SOUL, SPECTRE, SPIRIT, THEOLOGY, &c.

PNEUMATOMACHI, a fect of Chriftians in the 4th century, who opposed the proper divinity of the Holy Ghoft.

PNEUMATOSIS. See MEDICINE, Index. PNEUMONIA. See MEDICINE, and PHARMACY. Index.

PNIGEUS, an ancient town of Egypt, near Phoenicia. Strabo, xvi.

PNYX, a place in Athens, allotted by Solon for holding affemblies. C. Nep.

(1.) PO, a large and celebrated river of Italy, anciently called ERIDANUS, and PADUS, which rifes from Mount Vifo, in the ci-devant prov. of Piedmont, at the NW. part of the late marquifate of Saluzzo, 7 miles N. of Chateau Dauphin, on the borders of the late prov. of Dauphiny. It croffes Saluzzo, runs through the late provinces of Chieri, Piedmont, Montferrat, Milan, Mantua, Modena, and Ferrara; where it begins to divide at Fiche ruolo, and at laft falls into the Adriatic by 4 principal mouths. In its courfe, it paffes Villa Franca, Polonghera, Carmagnola, Carignano, Moncalier, Turin, Chivaflo, Verrua, Cafa, Brema, Valencia, Placentia, Cremona, Viadana, Borgoforte, St

Benedict,