Gambar halaman
PDF
ePub

sure.

371

to free

Effects of rents produced by heating the air is to the freeing mines, We shall conclude this part of cur subject with the

Precot Air's pre- ships, prisons, &c. from the damp and noxious vapours explanation of a curious plienomenon observed in many to jes which frequently infest them.

places. Certain springs or fountains are observed in As a drift or work is carried on in the mine, let a hare periods of repletion and scantiness, or seem to ebb73 Currents of trunk of deal boards, about 6 or 8 inches square, be and flow at regular intervals ; and some of these periods iets of the air applied laid along the bottom of the drift, communicating with are of a complicated nature.

Thus a well will have se air's poter a trunk carried up in the corner of one of the shafts. veral returns of high and low water, the difference of scie. mines, ships, pri

Let the top of this last trunk open into the ash-pit of a which gradually increases to a maximum, and then disons, &c. small furnace, having a tall chimney. Let fire be kin- minishes, just as we observe in the ocean. A very inof noxious dled in the furnace; and when it is well beated, shut genious and probable explanation of this has been given air. the fire-place and ash-pit doors. There being no other in N° 424. of the Philosophical Transactions, by Mr

supply for the current produced in the chimney of this Atwell, as follows.
furnace, the air will flow into it from the trunk, and Let ABCD (fig. 80.) represent a cavern, into wbich Fig fa.
will bring along with it all the offensive vapours. This water is brought by the subterraneous passage OT. Let
is the most effectual method yet found out. In the it have an outlet MNP, of a crooked form, with its
same manner may trunks be conducted into the ash-pit highest part N considerably raised above the bottom of

of a furnace from the cells of a prison or the wards of the cavern, and thence sloping downwards into lower 372 an hospital.

ground, and terminating in an open well at P. Let Air neces. In the account which we have been giving of the the dimensions of this canal be such that it will discharge sary for the combustion management of air in furnaces and common fires, we much more water than is supplied by TO. All this is of fuel.

have frequently mentioned the immediate application of very natural, and may be very common. The effect of
air to the burning fuel as necessary for its combustion. this arrangement will be a remitting spring at P: før
This is a general fact. In order that any inflammable when the cavern is filled higher than the point n, the
body may be really inflamed, and its combustible matter canal MNP will act as a syphon; and, by the condi-
consumed and ashes produced, it is not enough tbat the tions assumed, it will discharge the water faster than
body he made bot. A piece of charcoal inclosed in a TO supplies it; it will therefore run it dry, and then
box of iron may be kept red bot for ever, without wast- the spring at P will cease to furnish water. After some
ing its substance in the smallest degree. It is farther time the cavern will again be filled up to the height N,
necessary that it be in contact with a particular species and the flow at P will recommence.
of air, wbich constitutes about three-fourths of the air of If, besides this supply, the well P also receive water
the atmosphere, viz. the vital air or oxygen of Lavoisier. from a constant source, we shall have a reciprocating
It was called empyreal air by Scheele, who first observed spring.
its indispensable use in maintaining fire: and it appears, The situation and dimensions of this syphon canal,
that, in contributing to the combustion of an inflam- and the supply of the feeder, may be such, that the ef-
mable body, this air combines with some of its ingredi- flux at P will be constant. If the supply increase in a
ents, and becomes fixed air, suffering the same change as certain degree, a reciprocation will be produced at P
by the breathing of animals. Combustion may therefore with very short intervals; if the supply diminishes consi-
be considered as a solution of the inflammable body in derably, we shall have another kind of reciprocation
air. This doctrine was first promulgated by the cele- with great intervals and great differences of water.
brated Dr Hooke in his Micrographia, published in If the cavern have another simple outlet R, new va
1660, and afterwards improved in his treatise on Lamps. rieties will be produced in the spring P, and R will af-
It is now completely established, and considered as a new ford a copious spring. Let the mouth of R, by which
discovery. It is for this reason, that in fire-places of all the water enters into it from the cavern, be lower than
kinds we have directed the construction, so as to pro- N, and let the supply of the feeding spring be no great-
duce a close application of the air to the fuel. It is er than R can discharge, we shall have a constant spring
quite needless at this day to enter into the discussions from R, and P will give no water. But suppose

that
· which formerly occupied philosophers about the manner the main feeder increases in winter or in rainy seasons,
in which the pressure and elasticity of the air promoted but not so much as will supply both P and R, the ca-
combustion. Many experiments were made in the 17th vern will fill till the water gets over N, and R will be
century by the first members of the Royal Society,'to running all the while ; but soon after P has begun to
discover the office of air in combustion. It was thoughtflow, and the water in the cavern sinks below R, the
that the flame was extinguished in rare air for want of a stream from R will stop. The cavern will be emptied
pressure to keep it together; but this did not explain its by the syphon canal MNP, and then P will stop. The
extinction when the air was not renewed. These expe- cavern will then begin to fill, and when near full R
riments are still retained in courses of experimental will give a little water, and soon after P will run and R
philosophy, as they are judiciously styled; but they give stop as before, &c.
little or no information, nor tend to the illustration of Desagulier shows, vol. ii. p. 177, &c. in what man-
any pneumatical doctrine ; they are therefore omitted ner a prodigious variety of periodical ebbs and flows
in this place. In short, it is now fully established, that may be produced by underground canals, which are ex-
it is not a mechanical but a chemical phenomenon. We tremely simple and probable.
can only inform the chemist, that a candle will consume
faster in the low countries than in the elevated regions of We shall conclude this article with the descriptions
Quito and Gondar, because the air is nearly one half of some pneumatical machines or engines which bave
denser below, and will act proportionally faster in de- not been particularly noticed under their names in the matic es-
composing the candle.

[ocr errors]

former volumes of this work.

Bellows

574 Accuent SOBE

gia.

Fig. 22.

may

Pneumatic Bellows are of most extensive and important use; in this position by a number of steel wires, which are Pneumatic
Engines. and it will be of service to describe such as are of un- driven into the bottom of the box, and stand up touch- Engines.

common construction and great power, fit for the great ing the sides, as represented in hig. 95. wliere a b c are
operations in metallurgy.

the wires, and e the lath, projecting over the outside of Fig. 95.
It is not the impulsive force of the blast that is want- the box. By this contrivance the laths are pressed close
ed in most cases, but merely the copious supply of air, to the sides and curved end of the moveable box, and
to produce the rapid combustion of inflammable matter; the spring wires yield to all their inequalities. A bar
and the service would be better performed in general if of wood RS (fig. 92.) is fixed to the upper board, by
this could be done with moderate velocities, and an ex- which it is either raised by machinery, to sink again by
tended surface. What are called air-furnaces, where a its own weight, having an additional load laid on ii, or
considerable surface of inflammable matter is acted on it is forced downward by a crank or wiper of the ma-
at once by the current which the mere heat of the ex- chinery, and afterwards raised.
pended air has produced, are found more operative in The operation here is precisely similar to that of
proportion to the air expended than blast furnaces ani- blowing withi a chamber-bellows. When the board is
mated by bellows; and we doubt not but that the me- lifted up, the air enters by the valves V, V, fig. 94. and
thod proposed by Mr Cotterel, (which we have already is expelled at the pipe OQ by depressing the boards.
mentioned) of increasing this current in a melting fur- There is therefore no occasion to insist on this point.
nace by means of a dome, will in time supersede the These bellows are made of a very great size, AD
blast furnaces. There is indeed a great impulsive force being 16 feet, AB five feet, and the circular end AE
required in some cases ; as for blowing off the scoriæ also five feet. The rise, bowever, is but about 3 or 3.5
from the surface of silver or copper in refining furnaces, feet. They expel at each stroke about go cubic fiet
or for keeping a clear passage for the air in the great of air, and they make about 8 strokes per minute.
iron furnace.

Such are the bellows in general use on the continent. In general, however, we cannot procure this abun. We have adopted a different form in this kingdom, which dant supply of air any other way than by giving it a seems much preferable. We use an iron or wooden cygreat velocity by means of a great pressure, so that the linder, with a piston sliding along it. This be made general construction of bellows is pretty much the same with much greater accuracy than the wooden boxes, at in all kinds. The air is admitted into a very large ca- less expence, if of wood, because it

may

be of coopers vity, and then expelled from it through a small hole. work, held together by hoops; but the great advantage

The furnaces at the mines having been greatly en- of this form is its being more easily made air-tight. The larged, it was necessary to enlarge the bellows also : and piston is surrounded with a broad strap of thick and soft the leathern bellows becoming exceedingly expensive, leather, and it has around its edge a deep groove, in wooden ones were substituted in Germany about the be- which is lodged a quantity of wool. This is called the

ginning of the 17th century, and from them became packing or stufling, and keeps the leather very closely Plate ccccxxx.

general through Europe. They consist of a wooden box applied to the inner surface of the cylinder. Iron cylin

. tig. 92.

ABCPFE (fig. 92.), which has its top and two sides ders may be very neatly bored and smoothed, so that
flat or straight, and the end BAE e formed into an the piston, even when very tight, will slide along it very
arched or cylindrical surface, of wbich the line FP at smoothly. To promote this, a quantity of black lead is
the other end is the axis. This box is open below, and ground very fine with water, and a little of this is smear-
receives within it the shallow box KHGNML (fig. 93.), ed on the inside of the cylinder from time to time.
which exactly fills it. The line FP of the one coincides The cylinder has a large valve, or sometimes two, in
with FP of the other, and along this line is a set of the bottom, by which the atmospheric air enters when
hinges on which the upper box turns as it rises and sinks. the piston is drawn up. When the piston is thrust down,
The lower box is made fast to a frame fixed in the this air is expelled along a pipe of great diameter, which
ground. A pipe OQ proceeds from the end of it, and terminates in the furnace with a small orifice.
terminates at the furnace, where it ends in a small pipe This is the simplest form of bellows which can be
called the tewer or tuyere. This lower box is open conceived. It differs in nothing but size from the bel-

above, and has in its bottom two large valves V, V, lows used by the rudest nations. The Chinese smiths Fig 94.

fig. 94. opening inwards. The conducting pipe is some- have a bellows very similar, being a square pipe of wood
times furnished with a valve opening outwards, to pre- ABCDE (fig. 96.), with a square board G which ex- - Fig. 96.
vent burning coals from being sucked into the bellows actly fits it, moved by the handle FG. At the farther
when the upper box is drawn up. The joint along PF end is the blast pipe HK, and on each side of it a valve
is made tight by thin leather nailed along it. The sides in the end of the square pipe, opening inwards. The
and ends of the fixed box are made to fit the sides and piston is sufficiently tight for their purposes without any
curved end of the upper box, so that this last can be deathering.
raised and lowered round the joint FP without sensible The piston of this cylinder bellows is moved by ma-
friction, and yet without suflering much air to escape : chinery.

chinery. In some blast engines the piston is simply
but as this would not be sufficiently air-tight by reason raised by the machine, and then let go, and it descends
of the sbrinking and warping of the wood, a farther by its own weight, and compresses the air below it to
contrivance is adopted. A slender lath of wood, divided such a degree, that the velocity of efflux becomes con-
into several joints, and covered on the outer edge with stant, and the piston descends uniformly: for this pur-
very soft leather, is laid along the upper edges of the pose it must be loaded with a proper weight. This
sides and ends of the lower box. This lath is so broad, produces a very uniform blast, except at the very begin-
that when its inner edge is even with the inside of the ning, while the piston falls suddenly and compresses the
box, its outer edge projects about an inch. It is kept air: but in most engines the piston rod is forced down
Vol. XVI. Part II.

+

Fig. 93.

SB

the

Fig. 97.

Pneumatic the cylinder with a determined motion, by means of a When the engine worked briskly, it made 18 strokes Preumatie Engines. bean, crank, or other contrivance. This gives a more per minute, and there was always much air discharged Engioen.

unequal blast because the motion of the piston is neces- by the snifting valve. When the engine made 15 strokes sarily slow in the beginning and end of the stroke, and per minute, the snifting valve opened but seldon, so that quicker in the middle.

things were nearly adjusted to this sopply. Each stroke But in all it is plain that the blast must be desultory of the blowing cylinder sent in 118 cubic feet of comIt ceases while the piston is rising; for this reason it is mon air. The ordinary pressure of the air being supusual to have two cylinders, as it was formerly usual to posed 14 pounds on an inch, the density of the air in have two bellows, which worked alternately. Some

14.75 +2.63 times three or four are used, as at the Carron iron

the regulating cylinder must be

-=1.1783,

14.75 works. This makes the blast abundantly uniform. the natural density being 1.

But an uniform blast may be made with a single cy- This machine gives an opportunity of comparing the linder, by making it deliver its air into another cylin- expence of air with the theory. It must (at the rate der, which bas a piston exactly filted to its bore, and of 15 strokes) expel 30 cubic feet of air in a second loaded with a sufficient weight. The blowing cylinder through a bole of 1 inches in diameter. This gives ABCD (fig. 97.) has its piston P worked by a rod a velocity of near 2000 feet per second, and of more NP, connected by double chains with the arched head than 1600 feet for the condensed air.

This is vastly of the working beam NO, moving round a gudgeon at

greater than the theory can give, or is indeed possible; R. The other end of this beam is connected by the for air does not rush into a void with so great velocity. rod OP, with the crank PQ of a wheel machine ; or it It shows with great evidence, that a vast quantity of may be connected with the piston of a steam engine, air must escape round the two pistons. Their united &c. &c. The blowing cylinder bas a valve or valves circumferences amount to above 40 feet, and they move E in its bottom, opening inwards. There proceeds in a dry cylinder. It is impossible to prevent a very from it a large pipe CF, which enters the regulating great loss. Accordingly, a candle held near the edge cylinder GHKI, and has a valve at top to preveut the of the piston L has its flame very much disturbed. This air from getting back into the blowing cylinder. It is

case therefore gives no hold for a calculation ; and it evident that the air forced into the cylinder must raise

suggests the propriety of attempting to diminish this its piston L, and that it must afterwards descend, while

great waste. the other piston is rising. It must descend uniformly, This has been very ingeniously done (in part at least) and make a perfectly equable blast.

at some other furnaces. At Omoah foundry, near Observe, that if the piston L be at the bottom when Glasgow, the blowing cylinder (also worked by a steam the machine begins to work, it will be at the bottom

engine) delivers its air into a chest without a bottom, at the end of every stroke, if the tuyere T emits as which is immersed in a large cistern of water, and supmuch air. as the cylinder ABCD furnishes; nay, it will ported at a small height from the bottom of the cistern, lie a while at the bottom, for, while it was rising, air and has a pipe from its top leading to the tuyere. The was issuing through T. This would make an interrupt- water stands about five feet above the lower brim of the ed blast. To prevent this, the orifice T must be lessen- regulating air-chest, and by its pressure gives the most ed; but then there will be a surplus of air at the end of perfect uniformity of blast, without allowing a particle each stroke, and the piston L will rise continually, and

of air to get off by any other passage besides the tuyere. at last get to the top, and allow air to escape. It is This is a very effectual regulator, and must produce a just possible to adjust circumstances, so that neither

great saving of power, because a smaller blowing cylinshall happen. This is done easier by putting a stop in der will thus supply the blast. We must observe, that

way of the pistou, and putting a valve on the piston, the loss round the piston of the blowing cylinder reor on the conducting pipe KST, loaded with a weight mains undiminished. a little superior to the intended elasticity of the air in A blowing machine was erected many years ago at the cylinder. Therefore, when the piston is prevented Chastillon in France on a principle considerably differby the stop from rising, the snifting valve, as it is called, ent, and which must be perfectly air-tight throughis forced open, the superfluous air escapes, and the blast out. Two cylinders AB (fig. 98.), loaded with great pizze preserves its uniformity.

weights, were suspended at the end of the levers (1), Call It may be of use to give the dimensions of a machine moving round the gudgeon E. From the top F, G 15.30 of this kind, which has worked for some years at a very of each there was a large flexible pipe which united groat furnace, and given satisfaction.

in H, from whence a pipe KT led to the toyere T. The diameter of the blowing cylinder is 5 feet, and

There were valves at F and Gopening outwards, or the length of the stroke is 6. Its piston is loaded with into the flexible pipes, and other valves L, M, adjoin34 tons. It is worked by a steam engine whose cylin- ing to them in the top of each cylinder, opening inder is 3 feet 4 inches wide, with a six-feet stroke. The

wards, but kept shut by a slight spring. Motion was regulating cylinder is 8 feet wide, and its piston is load

given to the lever by a machine. The operation of this ed with 8: tons, making about 2.63 pounds on the square blowing machine is evident. When the cylinder A inch; and it is very nearly in equilibrio with the load

was pulled down, or allowed to descend, the water, enon the piston of the blowing cylinder. The conducting tering at its bottom, compressed the air, and forced it pipe KST is 12 inches in diameter, and the orifice of along the passage FHKT. In the mean time, the cy: the tuyere was of an inch when the engine was erect- Jinder B was rising, and the air entered by the valve M. ed, but it has gradually enlarged by reason of the in- We see that the blast will be very unequal, increasing tense heat to which it is exposed. The snifting valve as the cylinder is immersed deeper. It is needless to is loaded with 3 pounds on the square inch.

describe tbis machine more particularly, because we shall

the

Fig. 99.

Pneumatic give an account of one which we think perfect in its sage c, it will pass along the conduit cde to the tayere, Pneumatic Engines. kind, and which leaves hardly any thing to be desired and form a blast.

Engines.
in a machine of this sort. It was invented by Mr John The operation of this machine is similar to Mr Has-
Laurie, land-surveyor in Edinburgh, about 15 years kin's quicksilver pump described by Desaguliers at
ago, and improved in some respects since his death by the end of the second volume of his Experimental Phi-
an ingenious person of that city,

losophy. The force which condenses the air is the load
ABCD (fig. 99.) is an iron cylinder, truly bored on the middle cylinder. The use of the water between
within, and evasated a-top like a cup. EFGH is an- the inner and outer cylinders is to prevent this air from
other, truly turned both without and within, and a small escaping; and the inner cylinder tbus performs the office
matter less than the inner diameter of the first cylinder. of a piston, having no friction. It is necessary that
This cylinder is close above, and hangs from the end the length of the outer and middle cylinders be greater
of a lever moved by a machine. It is also loaded with than the depth of the regulator-cistern, that there
weights at N. KÍLM is a third cylinder, whose out- may be a sufficient height for the water to rise between
side diameter is somewhat less than the inside diameter the middle and outer cylinders, to balance the com-
of the second. This inner cylinder is fixed to the same pressed air, and oblige it to go into the air-chest. A
bottom with the outer cylinder. The middle cylinder large blast-furnace will require the regulator-cistern
is loose, and can move up and down between the outer five feet deep, and the cylinders about six or seven feet
and inner cylinders without rubbing on either of them. long.
The inner cylinder is perforated from top to bottom It is in fact a pump without friction, and is perfectly
by three pipes OR, SV, PR. The pipes OQ, PR air-tight. The quickness of its operation depends on
have valves at their upper ends O, P, and communicate the small space between the middle cylinder and the two
with the external air below. The pipe SV has a hori- others; and this is the only use of these two. With-
zontal part VW, which again turns upwards, and has out these it would be similar to the engine at Chastillon,
a valve at top X. This upright part WX is in the and operate more unequally and slowly: Its only im-
middle of a cistern of water fhkg. Into this cistern perfection is, that if the cylinder begin its motion of
is fixed an air-chest a YZb, open below, and having ascent or descent rapidly, as it will do when worked
at top a pipe cde terminating in the tuyere at the fur- by a steam-engine, there will be some danger of water
nace.

dashing over the top of the inner cylinder and getting
When the machine is at rest, the valves X, O, P, into the pipe SV; but should this bappen, an issue can
are shut by their own weights, and the air-chest is full easily be contrived for it at V, covered with a loaded
of water. When things are in this state, the middle valve i. This will never happen if the cylinder is
cylinder EFGH is drawn up by the machinery till its moved by a crank.
lower brims F and G are equal with the top RM of the One blowing cylinder only is represented here, but
inner cylinder. Now pour in water or oil between the two may be used.
outer and middle cylinders : it will run down and fill We do not hesitate in recommending this form of
the space between the outer and inner cylinders. Let bellows as the most perfect of any, and fit for all uses
it come to the top of the inner cylinder.

where standing bellows are required. They will be
Now let the loaded middle cylinder descend. It cheaper than any other sort for common purposes. For
cannot do this without compressing the air which is a common smith's forge they may be made with square
between its top and the top of the inner cylinder. This wooden boxes instead of cylinders They are also easily
air being compressed will cause the water to descend be- repaired. They are perfectly tight; and they may be
tween the inner and middle cylinders, and rise between made with a blast almost perfectly uniform, by making
the middle and outer cylinders, spreading into the cup; the cistern in which the air-chest stands of considerable
and as the middle cylinder advances downwards, the dimensions. When this is the case, the height of water,
water will descend farther within it, and rise farther which regulates the blast, will vary very little.
without it. When it has got so far down, and the air This may suflice for an account of blast machines.
has been so much compressed, that the difference between The leading parts of their construction have been de-
the surface of the water on the inside and outside of scribed as far only as was necessary for understanding
this cylinder is greater than the depth of water between their operation, and enabling an engineer to erect them
X and the surface of the water fg, air will go out by in the most commodious manner. Views of complete
the pipe SVW, and will lodge in the air-chest, and machines might have amused, but they would not have
will remain there if c be shut, which we shall suppose added to our reader's information.
for the present. Pushing down the middle cylinder But the account is imperfect unless we show how
till the partition touch the top of the inner cylinder, their parts may be so proportioned that they shall per-
all the air which was formerly between them will be form what is expected from them. The engineer should
forced into the air chest, and will drive out water from know what size of bellows, and what load on the board
it. Draw up the middle cylinder, and the external or piston, and what size of tuyere, will give the blast
air will open the valves O, P, and again fill the space which the service requires, and what force must be em.
between the middle and inner cylinders ; for the valve ployed to give them the necessary degree of motion.
X will shut, and prevent the regress of the condensed We shall accomplish these purposes by considering the
air. By pushing down the middle cylinder a second efflux of the compressed air through the tuyere. The
time, more air will be forced into the air-chest, and it propositions formerly delivered will enable us to ascer-
will at last escape by getting out between its brims. Y, tain this.
Z and the bottom of the cistern; or if we open the pas- That we may proportion every thing to the power

5 B 2

employed,

[ocr errors]

v=Vx

[ocr errors]

PXP

powers. It

+

P+P

Pneumatic employed, we must recollect, that if the piston of a cy- sed the air) descend about 15 inches in a second : It Pneumatie.
Engines, linder employed for expelling air be pressed down with would first sink one-fifth of the whole length of the cy. Engiaes

any
force

P, it must be considered as superadded to the linder pretty suddenly, till it bad reduced the air to the
atmospheric pressure P on the same piston, in order that density if, and would then descend uniformly at the
we may compare the velocity v of efflux with the known above rate, expelling six cubic feet of common air in a
velocity V with which air rushes into a void. By what second.
has been formerly delivered, it appears that this velocity The computation is made much in the same way for

bellows of the common form, with this additional cir. where P is the pressure of the atmo- cumstance, that as the loaded board moves round a sphere on the piston, and

hinge at one end, the pressure of the load must be calcuthe additional load laid on it. P

lated accordingly. The computation, however, becomes This velocity is expressed in feet per second; and, when multiplied by the area of the orifice (also expressed in

a little intricate, when the form of the loaded board is

not rectangular: it is almost useless when the bellows square feet, it will give us the cubical feet of conden

have flexible sides, either like smiths bellows or like sed air expelled in a second: but the bellows are always to be filled again with common air, and therefore

organ bellows, because the change of figure during their

motion makes continual variation on the compressing we want to know the quantity of common air which

therefore chiefly with respect to the will be expelled ; for it is this which determines the number of strokes which must be made in a minute, in

great wooden bellows, of which the upper board slides

down between the sides, that the above calculation is of order that the proper supply may be obtained. There

service. fore recollect that the quantity expelled from a given orifice with a given velocity, is in the proportion of the is evident: we do not know precisely the quantity of

The propriety, however, of this piece of information density; and that when D is the density of common air produced by the pressure P, the density d produced by tion tells us what force must be employed for expelling

air necessary for animating a furnace; but this calculathe pressure P+1, is Dx P+P; or if d be made 1,

the air that may be thought necessary. If we have fixed

ou the strength of the blast, and the diameter of the cywe have de

linder, we learn the weight with which the piston must P

be loaded; the length of the cylinder determines its caTherefore, calling the area of the orifice expressed in pacity, the above calculation tells the expence per sesquare feet 0, and the quantity of common air, or the cond; hence we have the time of the piston's coming cubic feet expelled in a second Q, we have Q=V XOX to the bottom. This gives us the number of strokes per

minute : the load must be lifted up by the machine this P X

number of times, making the time of ascent precisely It will be sufficiently exact for all practical purposes equal to that of descent;

otherwise the machine will ei. to suppose P to be 15 pounds on every square inch of

ther catch and stop the descent of the piston, or allow it

to lie inactive for a while of each stroke. These circum. the piston ; and p is then conveniently expressed by the pounds of additional load on every square inch : we may chine, and it must be constructed accordingly. Thus

stances determine the labour to be performed by the maalso take V=1332 feet. As the orifice through wbich the air is expelled is ge

the engineer will not be affronted by its failure, nor will nerally very small, never exceeding three inches in dia

be expend needless power and cost. meter, it will be more convenient to express it in square

In machines which force the piston or bellows-board inches; which being the rit of a square foot, we shall

with a certain determined motion, different from what have the cubic feet of common air expelled in a second, or

arises from their own weight, the computation is ex.

tremely intricate. When a piston moves by a crank, its P P+1

P
Q=

motion and end ,
Х
144

and the efflux : X

we can however approximate to a statement of the force and this seems to be as simple an expression required. P as we can obtain.

Every time the piston is drawn up, a certain space This will perhaps be illustrated by taking an example density, and this is expelled during the descent of the

of the cylinder is filled again with air of the common in numbers. Let the area of the piston be four feet, and the area of the round hole through which the piston. A certain number of cubic feet of common air air is expelled be two inches, its diameter being 1.6, tinually varies; but there is a medium velocity with which

is therefore expelled with a velocity which perhaps conand let the load on the piston be 1728 pounds : this is three pounds on every square inch.

it might have been uniformly expelled, and a pressure

=. p=3, P+p=18, and O=2; therefore we will have corresponding to this velocity. To find this, divide the

area of the piston by the area of the blast-bole (or ra. 3 18 Q=2X9.25 XA X

ther by this area multiplied by 0.613, in order to take =9.053 cubic feet of com18 15'

in the effect of the contracted jet), and multiply the mon air expelled in a second. This will however be di- length of the stroke performed in a second by the quominished at least one-third by the contraction of the jet; tient arising from this division ; the product is the meand therefore the supply will not exceed six cubic feet dium velocity of the air (of the natural density). Theo per second. Supposing therefore that this blowing ma- find by calculation the height through wbich a heavy chine is a cylinder or prism of this dimension in its sec- body must fall in order to acquire this velocity; this is tion, the piston so loaded would (after baving compres- the height of a column of homogeneous air which would

P+P

P+P

13320

P+P

[ocr errors]
« SebelumnyaLanjutkan »