I next turn the wheel till the tubes E² and E⁴ are in a vertical position, by which turning the mercury therein is forced into the tube E³, flowing down which it drives the piston G³ of the cylinder F³ forward to the extremity of its range, leaving a vacuum in the cylinder A¹ at O, equal to the difference between the heights from which the mercury descends in the tubes E¹ and E³. I then fill the tube E² and cylinder B² with mercury, to the same extent and in the same way as I previously filled the tube E¹ and cylinder A¹, after which I turn the wheel till the tubes E¹ and E³ are once more in a vertical position, whereby I produce a vacuum in each pair of tubes, and their intermediate cylinder, to the degree of the difference before explained. To the four tubes there are attached four cocks K¹, K², K³, K⁴, which, after the vacua have been obtained, are closed; and to the four rods of the pistons of the outer cylinders F¹, F², F³, F⁴, there are attached four hanging or balance weights L¹, L², L³, L⁴, in such manner that they shall co-operate with the atmospheric pressure on the said vacua in giving rotation to the wheel. M¹, M², M³, M⁴, are jointed levers, by which these weights are connected at one end with the pistons G¹, G², G³, G⁴; and N¹, N², are cords or bands, by which they are suspended at the other end from standards P, P, projecting from the ring R, and bearing pulleys, over which the cords or bands pass, each cord or band serving to suspend the two weights which are opposite to each other, for which purpose it is passed internally across the wheel and over the exterior of one of the cylinders A¹ or B¹. The cords or bands are attached to the weights at the lower ends thereof, and pass over small pulleys close to the points of connection, so that the cords or bands, when pulled, may act the more effectually on the weights. It will be readily understood that when any two of the tubes are in a vertical position, and the mercury or other suitable fluid has descended to the bottom of the lower tube, its pressure on the piston of the outer cylinder G¹, or G², or G³, or G⁴, will cause the weight connected with that piston to turn inwards towards the centre of the wheel, by which movement a strain is exerted on the connecting cord or band N¹ or N², which throws up the opposite weight at top, and causes it to force down the piston of the top cylinder, or the surface of the mercury in the upper tube, whereby any excess of pressure at the bottom of the lower tube is transferred to the top piston, where it acts in aid of the atmospheric pressure on the vacua obtained in manner aforesaid. The four cocks K, have regulating rods connected to them in the way common in steam and other engines, so that as each tube comes into a vertical position the cock attached to it is opened, and as it passes from that