Since this is counteracted by the pressure of the compressed air, which is to that of the atmosphere :: vol. AD: vol. AH, we must have

a quadratic equation from which z may be determined.

The tension of the string FE is equal to the weight of the bell and the inclosed air, minus that of the water displaced; or if T be this tension, W the weight of the bell, w that of the inclosed air, and A be the area of the horizontal section of the cylinder,

TW+w-σA.BH.

In practice a flexible tube is passed down to the bell under its lower edge, and air is forced through it from a condenser, so that the surface of the water GH is kept at any desired level and not allowed to rise to an inconvenient height. Other tubes provided with valves are also employed, by the aid of which the air may be changed when it is unfit for respiration.

The Atmospheric Engine.*

55. The annexed diagram represents a section of the Atmospheric Engine invented by Newcomen in the year 1705, for working the pumps of mines.

AB is a massive wooden beam (turning about an axis C, strongly supported), having its extremities terminated by circular arcs, which are connected by chains, the one (4) to the pump-rods (loaded, if necessary), the other to the rod of

*For the following description of the Atmospheric and Double-Action Steam Engine I am indebted to the kindness of a friend.

a solid piston E working steam-tight in an accurately bored cylinder F. This cylinder is open at the top, but closed at

the bottom, in which are the orifices of three tubes, H, G, K, each furnished with a cock, and passing, respectively, into the boiler, an elevated cistern of cold water and a waste pipe. Suppose steam to be generated in the boiler, the three cocks all closed and the piston at the bottom of the cylinder, it will be kept in this position by the atmospheric pressure on its upper surface. Let the cock H be opened, steam from the boiler will enter the cylinder below the piston and counterbalance the atmospheric pressure on its upper surface. The weight of the pump-rods being now unsupported will depress the extremity A of the beam and raise the piston to the top of the cylinder: the cock H is now closed and G opened, through which a jet of cold water rushing into the cylinder condenses the steam and forms a vacuum, more or less perfect, below the piston which is now driven down by the

atmospheric pressure on its upper surface, raising in its descent the pump-rods connected with A together with their load of water; the cock G is now closed, and the condensed steam and water let off by the cock K which is then also closed. The machine has now completed one stroke and is in the same condition as at first. Hence the operation may be repeated at pleasure.

The cocks H, G, K, originally turned by hand, were, by a contrivance of a youth named Potter, afterwards worked by the machine itself.

Watt's Improvements.

56. Such was the engine which came under the observation of James Watt, whose comprehensive genius perceiving its various defects, suggested amendments so complete as to bring it almost to the perfection of the beautiful engines of the present day. The following is an outline of his most material improvements.

The source of the motive power is the heat which is applied to the water in the boiler, and which calls into play the elastic force of the steam; and the real expense of working the machine is caused by the consumption of fuel required for generating this heat. It occurred to Watt that a great useless expenditure of heat was entailed by the foregoing method of condensing the steam at the end of each stroke; for while it is only wanted to cool the steam itself, the jet of cold water evidently lowered the temperature of the cylinder also, and therefore caused it every time to abstract a portion of the heat of the newly-introduced steam. To obviate this

he added a separate vessel (the condenser), in which the operation of condensing the steam might be performed, so that the cylinder should remain constantly of the same tempera

ture.

A further saving was also effected by pumping the contents of the condenser back to the boiler.

He now closed the cylinder at the top, and admitting steam alternately above and below the piston, converted the atmospheric into the double-action steam-engine.

He also invented that beautiful contrivance, the parallel motion, for keeping the extremity of the piston-rod in the same vertical line, while the end of the beam with which it is connected describes an arc of a circle.

The Double-action Condensing Engine.

57. The annexed diagram represents a section of a doubleaction condensing Engine, in its simplest form.

A is a tube by which steam is conveyed from the boiler to the steam-box B, which is a closed chamber having its side adjacent to the cylinder truly flat. In this side are the apertures of three tubes E, F, G, of which the two former enter the cylinder at the top and bottom respectively; the third passes into the condenser L; C is the slide-valve, being a piece of metal having one side accurately flat so as to slide in steam-tight contact with the flat face of the steam-box. In this face of the slide is cut a groove in length not greater than the distance between the apertures E, F, diminished by the width of the aperture, as in fig. (2). The rod of the slidevalve passes through a steam-tight collar in the bottom of the steam-box, and is connected by a lever OP and a rod PQ

with the beam of the engine, by which it receives a motion the reverse of that of the piston-rods. D is the cylinder

closed at both ends, truly bored and accurately fitted with a solid piston whose rod H works in a steam-tight collar in the top of the cylinder, and is connected with the beam by the parallel motion KaR. RS is the beam turning about an axis T and having its extremity S connected by a rod and crank with an axle on which is the fly-wheel W. L is the condenser, a closed vessel in which a jet of cold water is constantly playing. Mis a force-pump, worked by a rod connected to the beam, which returns the condensed steam and water from the condenser by the tube N to the boiler.