The alternate action of each cylinder is produced by chains and rods, attached to a glass or iron vessel p, more than half filled with mercury, and turning upon a pivot; each end receives its movements of elevation and depression from the rise and fall of the projecting arms q, by the action of the beam above; the mercury within flowing to the lower end, giving an impetus, and thus regulating the supply of gas to the cylinders, and the movement of the slide in the trough v. By this action the water from the reservoir flows down the pipe w, into the vessel x, and produces the elevation of the float y and the rod n, and raises the cap e by the ascent of the beam at a.
The motion thus produced in one part of the machinery, operates upon the corresponding parts on the other side, and hence a corresponding motion is obtained: the slider in the trough v, moved by the action of the mercurial tube p, being removed from its position, allows the water to fall into the other pipe w; and, as it ascends, suffers the float y to descend, and rising into the main cylinder, then lifts again the beam at b, and its connexions, and forces down the cap e on the top of the other cylinder.
When the vacuum is produced in the cylinders, the air must be admitted to allow the water to be discharged, and the caps to be raised: this is effected by a sliding valve in the air-pipe m m, acted upon by chains t t, attached to the floats in the reservoir, and as motion is given to them, the valve is made to fly backwards and forwards, so as to allow the free admission of atmospheric air.
Chains u u, with suspended weights, open the cocks in the pipe k k, and produce the alternate flow of the gas, and regulate and modify its supply. In the pipes g i g, and h j h, are clacks to prevent the return of the water, when the air is admitted into the cylinders.
A piston may be worked as is above described, with the machinery attached; but it may also be worked in a distinct vessel so as to communicate with several cylinders, and, consequently, several pistons may work at the same time, the air and vacuum valves being opened and closed by similar means to those adapted to work the induction and eduction valves of steam-engines.
The atmospheric engine comes next in order, and its claim to practical utility is of a very early date.
The cylinder b, is in this engine placed over a boiler n, and if we suppose the piston p made to fit air-tight, it will be evident, that it must be driven up by the action of the steam beneath, should a sufficient supply of heat be applied; when this is effected, the condensible vapour may be reduced to its original bulk, by the introduction of water from the cistern i. In the working engine however, the ascent of the piston is effected by the action of the lever e g, acting on the fulcrum f. To the end g of this lever or working beam is attached the pump-rod h, and it will be evident that whenever that preponderates over the piston p, that the latter must be drawn up. On the readmission of the steam, a new supply of condensing water is introduced by turning the cock l, and the pressure of the atmosphere above the piston being unbalanced by any resistance beneath, the end e is again depressed, and the pump-rod again elevated. The pipe g is employed to carry off the condensing water, which would otherwise accumulate within the cylinder; and the small forcing pump, with its rod v s, supplies the condensing cistern i, by the pipe t.
At the beginning of the last century, the atmospheric engine had made considerable progress in the mining districts, and in 1718, the patentees agreed to erect an engine for the owners of a colliery, in the county of Durham, where several hundred horses had previously been employed. Mr. Henry Beighton, who was engaged as an agent in this concern, materially improved the engine by making it self-acting, and divesting it of nearly all the complicated machinery, which had been previously employed for that purpose.