brought into the hot-well g, is pumped up again by the pump h, and is brought back again into the boiler by the pipe ik is another pump, also worked by the engine itself, which supplies the cistern in which the condenser is placed, with cold water.

Of Pumps.

401. The pump is a common and most useful engine, first invented by Ctesibius, a mathematician of Alexandria, about 120 years B. C. When the pressure of the air came afterwards to be known, it was improved; and it is now brought to great perfection.

Of this machine there are three kinds, the sucking, the lifting, and the forcing pump.

By the two last, water may be raised to any height, with an adequate apparatus and sufficient power; by the sucking pump, it can only be raised thirty-three feet above the surface of the water. In practice, however, this kind of pump is seldom applied to raise water much above twenty-eight feet; because the air is sometimes lighter than thirty-three feet of water; and whenever that is the case, the pump will fail to act.

402. The sucking-pump is an engine both pneumatic and hydraulic :

E

Consisting of a pipe A B, open at both ends, in which is a moveable cylinder or piston C, as big as the bore of the pipe in that part wherein it works; and contrived by leathers or other means to fit the bore exactly, so as not to allow any air to pass between it and the sides of the pipe where it acts. In the piston there is a valve opening upwards, like a trap-door, to allow the air and water readily to ascend, but to prevent either of them from descending. This piston is called the bucket. It is moved up and down in the pipe by a rod fastened to a handle or lever, or such parts of machinery as are to work it. The pipe usually consists of two parts, of which the first and wider part A D, is called the working-barrel, because it contains the piston; and the other D B, the suction-pipe.

D

At the joining of the working barrel with the suction-pipe, there is a fixed valve D, opening also upwards. The lower end of the suction-pipe is immersed in water, and admitted into it through small holes at B, to prevent the entrance of dirt; at the top of the working-barrel is a wide head, and a pipe E, for the delivery of the water that is raised.

The water we admit is raised from the well by the pressure of the atmosphere. At the beginning of the operation, if the leathers be dry, the piston C, will not exhaust the air suffici ently, and the water will not rise; if a little water be poured upon the piston, it will swell the leathers and causing them to fit close, the piston will act. This is vulgarly called, fetching the water. The perpendicular height of the piston or bucket from the surface of the water in the well, must always be less than thirty-three feet, else the water will never get above the bucket. But when the height is less, the pressure of the atmosphere will be greater than the weight of the water in the pump, and will raise it above the bucket; and when the water has once got above the bucket, it may be lifted to any height, if the piston-rod be long enough, and a sufficient degree of strength employed.

403. In the lifting pump there is always a column of water lifted, whose base is equal to the top of the piston, and whose height is equal to the distance from the piston to the head. This weight will not be made less by diminishing the diameter of the barrel above the piston, because fluids press in proportion to their bases and perpendicular altitudes. This pump is much used in great waterworks; it is the simplest of all in its operation, and produces an influx by the spout nearly equable.

404. The forcing-pump, consists of a barrel, and a piston or forcer, together with a main or lateral pipe to deliver the water, and in which there is a valve. The great end of the forcing pump is to convey the water further from its bed than either the sucking or lifting pump; and this it does more easily, by means of the lateral pipe and valve.

Thus, when the piston or plunger G is lifted up by the rod D, the water forces up the valve b in the box H, and rises into the body or barrel of the pump above H. When the piston G, therefore, (which we must observe has no hole or

valve in it) is depressed to H, the valve b being closed by this action; the water in the barrel of the pump finding no other vent is forced into the pipe MM, and so up through the pipe. If there is no occasion for a continued stream of water, the pipe M is continued to any given height, and then the water would be thrown out like a jet d'eaux at every stroke of the piston.

But to make a continued stream, a further contrivance is necessary, and the air vessel KK has been annexed to the pipe M, to receive the water forced into it by each stroke of the piston. When, therefore, the water by this action continued, gets above the lower end of the pipe GHI, which is fixed air, tight in the top of the vessel, the air in the upper part is proportionably condensed. The action of the pump being then continued, in proportion as the vessel KK is filled with water, the air above is compressed, and in return presses in the surface of the water, and drives it out through the pipe at the orifice G in its end in a continued stream, and with great force.

The fire engine is nothing more than two of these pumps in action, to produce one continued stream.

K

405. The chain pump consists of two square, or cylindrical barrels, through which a chain passes, having a great number of flat pistons, or valves, fixed upon it at proper distances.

This chain passes round a kind of wheel-work, fixed at one end of the machine. The teeth of this are so contrived as to receive one half of the flat pistons, which go free of the sides of the barrel by near a quarter of an inch, and let them fold in, and they take hold of the links as they rise. A whole row of the pistons, which go free of the sides of the barrel by near a quarter of an inch, are always lifting when the pump is at work, and as this machine is generally worked with briskness, they bring up a full bore of water in the pump. It is wrought either by one or two handles, according to the labour required.

The preference which has been given to chain-pumps, over those which work by the pressure of the atmosphere, must have arisen from this circumstance, that the former have been found less liable to choke.

In point of friction, of coolness, and of cheapness, the sucking-pump has so evidently the advantage over the chainpump, that it will not fail to gain the preference, when it shall be no longer liable to be choaked with gravel, &c. &c.

406. Buchanan's pump, which, like the common pump, acts by the pressure of the atmosphere, is not liable to the defects incident to other pumps upon that principle, being essentially different from those in general use. The principal object of its invention was to remove the imperfection of choaking, and in attaining this end, collateral advantages have also been produced, which enhance its utility. The points in which it differs from the common pump, and by which it excels, are, that it discharges the water below the piston, and has its valves lying near each other.

The advantages of this arrangement are, 1st, that the sand which may be in the water, is discharged without injuring the barrel or the piston-leathers; so that besides avoiding unnecessary tear and wear, the power of the pump is preserved, and it is not apt to be diminished or destroyed in moments of danger, as is often the case with the common and chain-pumps; that the valves are not confined to any particular dimensions, but may be made capable of discharging every thing that can rise in the suction-piece, without danger of being choaked; 2dly, if, upon any occasion, there should happen to be an obstruction in the valves, they are both within the reach of a person's hand, and may be cleared at once, without the disjunction of any part of the pump; and, 3dly, the pump is rendered capable of being instantaneously converted into an engine for extinguishing fire. Besides, it occupies very little space in the hold of a ship, and thus saves room for stowage.

Mr. Buchanan invented this pump when superintending the cotton mills at Rothsay; and though then a very little boy, I accompanied my father to see the first exhibition of this useful engine; indeed, I have every reason to believe that Mr. Buchanan consulted my father in this invention.

But this pump is not confined to nautical uses alone; its

adaption extends to the raising of water in all situations, and with peculiar advantage where it happens to be mixed with sand or substances which destroy other pumps, as, for instance, in alum-works, in mines, in quarries, in the clearing of foundations; and, in its double capacity, it may be very conveniently used in gardens, bleaching-grounds, in stable and farm-yards, and in all manufactories, or other places, where there is a necessity for raising water, and the risk of fire.

With all these advantages, it is a simple and durable pump, and may be made either of metal or wood, at a moderate expense.

The Diving Bell.

407. To illustrate the principle of this machine, take a glass tumbler, plunge it into water with the mouth downwards; you will find that very little water will rise into the tumbler; which will be evident, if you lay a piece of cork upon the surface of the water, and put the tumbler over it; for you will see, that though the cork should be carried far below the surface of the water, yet that its upper side is not wetted; the air which was in the tumbler having prevented the entrance of the wa

ter; but as air is compressible, it could not entirely exclude the water, which, by its pressure, condensed the air a little.

The first diving-bell of any note, was made by Dr. Halley. It is most com monly made in the form of a truncated cone, the smallest end being closed and the larger one open. It is weighted with lead, and so suspended that it may sink full of air, with its open base downwards, and as near as may be parallel to the horizon, so as to close with the surface of the water. Mr. Smeaton's diving bell was a square chest of cast iron,

four feet and a half in height, four feet and a half in length, and three feet wide, and afforded room for two men to work in it. It was supplied with fresh air by a forcingpump. This was used with great success at Ramsgate. Other