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. [Illustration]