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OF THE PUMP.
380. THERE are three sorts
D of pumps : the Sucking, the Lifting, and the Forcing Pump. A By the first, water can be raised only to about 34 feet, viz. by
L the pressure of the atmosphere; but by the others, to any height; but then they require more apparatus and power.
The annexed figure represents a common sucking pump is the barrel of the pump, being a hollow cylinder, made of metal, and smooth within, or of
OK wood for very common purposes CD is the handle, moveable about the pin E, by moving
B the end c up and down. an iron rod turning about a pin D, which connects it to the end of the handle. This rod is fixed to the piston, bucket, or sucker, rg, by which this is moved up and down within the barrel, which it must til very tight and close, that no air or water may pass between the piston and the sides of the barrel ; and for this purpose it is commonly armed with leather. The piston in made hollow, or it has a perforation through it, the oritice of which is covered by a valve u opening upwards. I is a plug firmly fixed in the lower part of the barrel, also perforated, and covered by a valve kopen. ing upwards.
381. When the pump is first to be worked, and the water is below the plug i; raise the end c of the handle, then the piston descending, compresses the air in hi, which by its spring shuts fast the valve K, and pushes up the valve H, and so enters into the barrel above the piston. Then putting the end c of the handle down again, raises the piston or sucker, which lifts up with it the column of air above it, the external atmosphere by its pressure keeping the valve u shut : the air in the barrel being thus exhausted, or rarefied, is no longer a counterpoise to that which presses on the sur. face of the water in the well ; this is forced up the pipe, and through the valve k, into the barrel of the pump. Then pushing the piston down again into this water, now in the
barrel its weight shuts the lower valve K, and its resistance forces up the valve of the piston, and enters the upper part of the barrel, above the piston. Then, the bucket being raised, lifts up with it the water which had passed above its valve, and it runs out by the cock L; and taking off the weight below it, the pressure of the external atmosphere on the water in the well again forces it up through the pipe and lower valve close to the piston, all the way as it ascends, thus keeping the barrel always full of water. And thus by repealing the strokes of the piston, a continued discharge is made at the cock L.
OF THE AIR PUMP.
382. NEARLY on the same principles as the water pump, is the invention of the Air-pump, by which the air is drawn out of any vessel, like as water is drawn out by the former. A brass barrel is bored and polished truly cylindrical, and exacıly fitted with a turned piston, so that no air can pass by the sides of it, and furnished with a proper valve opening upward. Then by lifting up the piston, the air in the close vessel below it follows the piston, and fills the barrel ; and being thus diffused through a larger space than before, when It occupied the vessel or receiver only, but not the barrel, it is made rarer than it was before, in proportion as the capacity of the barrel and receiver together exceeds the receiver alone Another stroke of the piston exhausts another barrel of this now rarer air, which, again rarefies it in the same proportion as before. And so on, for any number of strokes of the piston, still exhausting in the same geometrical progression, of which the ratio is that which the capacity of the receiver and barrel together exceeds the receiver, till this is exhausted to any proposed degree, or as far as the nature of the machine is capable of performing; which happens when the elasticity of the included air is so far diminished, by rarefying, that it is too feeble to push up the valve of the piston, and escape.
383. From the nature of this exhausting, in geometrical progression, we may easily find how much the air in the receiver is rarefied by any number of strokes of the piston ; or what number of such strokes is necessary, to exhaust the receiver to any given degree. Thus, it the capacity of the receiver and barrel together, be to that of the receiver alone,
as c tor, and I denote the natural density of the air at first : then
C:r::1: the density after one stroke of the piston,
&c. and the density after n strokes. So, if the barrel be equal to 1 of the receiver ; then c:q:: 5 : 4 ; and
= 081 is = d the density after a turns. And
50 if n be 20, then 0.820 = 0,15 is the density of the included air after 20 strok's of the piston; which being the 867 part of 1, or the first density, it follows that the air is 8610 times rarefied by the 20 strokes.
384. Or, if it were required to find the number of strokes necessary to rarely the air any number of times ; because
is = the proposed density d; therefore, taking the logarithms, n * log
log.d = log. d, and n=
Ir-lo' ber of strokes required. So if u be of c, and it be required to rarify the air 100 times: then d = Tóg or 0.1;
los. 100 and hence n =
20 nearly. So that in 203 1. 5strokes the air will be rarefied 100 times.
OF THE DIVING BELL & CONDENSING MACHINE.
385. Othe same principles too depend the opperations and effect of the Condensing Engine, by which air may be condensed to any degree instead of rarefied as in the air. pump. And, like as the air-pump rarefies the air, by ex. tracting always one barrel of air after another ; so, by this other machine, the air is condensed, by throwing in or adding always one barrel of air after another ; which it is evident
may be done by only turning the valves of the piston and barrel, that is, making them to open the conirary way, and working the piston in the same manner;
so that, as they both open upward or outward in the air-pump or rarefier, they will both open downward or inward in the condenser.
386. And on the same principles, namely, of the compression and elasticity of the air, depends the use of the Diving Bell, which is a large vessel, in which a person descends to the bottom of the sea, the open end of the vessel being downward ; only in this case the air is not condensed by forcing more of it into the same space, as in the condens• ing engine ; but by compressing the same quantity of air
into a less space in the bell, by increasing always the force which compresses it.
387. If a vessel of any sort be inverted into water, and pushed or let down to any depth in it ; then by the pressure of the water some of it will ascend into tlie vessel, but not so high as the water without, and will cori.press the air into less space, according to the difference between the heights of the internal and external water ; and the density and elastic force of the air will be increased in the same proportion, as. its space in the vessel is diminished.
So, if the tube ce be inverted, and pushed down into water, till the external water exceed the internal, by the height ab, and the air of the tube be reduced to the space CD ; then that air is pressed both by a column of water of the height ab, and by the
B the weight of the atmosphere, is to the weights both of the atmosphere and the column of water AB. So that if al be
Р about 34 feet, which is equal to the force of the atmosphere, then cd will be equal to fce; but if as be double of that, or 68 feet, then cd will be ice; and so on. And hence, by knowing the depth AF, 10 which the vessel is sunk, we can easily find the point D, to which the water will rise within it at any time. For let the weight of the atmosphere at that time be equal to that of 34 feet of water; also, let the depth AF be 20 feet, and the length of the tube CE 4 feet: then putting the height of the internal water De = x,
it is 34 + AB : 34 :: CE: CD,
= 136, and the root is x = v 2 very nearly = 1'414 of a foot, or 17 inches nearly ; being the height De to which the water wili rise within the tube.
388. But if the vessel be not equally wide throughout, but of any other shape, as of a bell-like form, such as is used in diving ; then the altitudes
A will not observe the proportion above, buc the spaces or bulks only will re. G spect that proportion, namely, 34 + B AB : 34 : capacity CKL
: capacity chr, if it be common or fresh water; Ek and 33 + AB : 33 : : capacity CKE : capacity chi, if it be sea-water. From which proportion, the height de may be found, when the nature or shape of the vessel or bell CIL is known.
OF THE BAROMETER.
389. THE BAROMETER is an instrument for measuring the pressure of the atmosphere, and elasticity of the air, at any time. It is commonly made of a glass tube, of near 3 feet long, close at one end, and filled with mercury. Waen the tube is full, by stopping the open end with the finger, then inverting the tube, and immersing that end with the finger into a bason of quicksilver, on removing the finger from the orifice, the fluid in the tube will descend into the bason, till what remains in the tube be of the same weight with a column of the atmosphere, which is commonly between 28 and 31 inches of quicksilver ; and leav. ing an entire vacuum in the upper end of the tube above the mercury. For, as the upper end of the tube is quite void of air, there is no pressure downwards but from the column of quicksilver, and therefore that will be an exact balance to the counter pressure of the whole column of atmosphere, acting on the orifice of the tube by the quicksilver in the bason. The upper 3 inches of the tube, namely; from 28 to 31 inches, have a scale attached to them, divided into inches, tenths, and hundredths, for measuring the length of the column at all times, by observing which division of the scale the top of the quicksilver is opposite to ; as it ascends and descends within ihese limits according to the state of the atmosphere.