It is obvious, however, that whether one cubic inch of sponge required one, two, or four ounces for this purpose, it would not affect the equilibrium, since, whatever were the proportion on the ascending side, precisely the same would the proportion be on the descending side. This principle is capable of application in various ways, and with a variety of materials. It may be produced by a single roller or wheel. Mercury may also be substituted for water, by using a series of metallic plates instead of sponges; and, as the mercury will be found to rise to a much greater height between these plates, than water will do in a sponge, it will be found that the power to be obtained by the latter materials will be from 70 to 80 times as great as by the use of water. Thus, a machine, of the same dimensions as given above, would have a constant power of 2,000 lbs. acting upon it. We now proceed to show how the principle of perpetual motion proposed by Sir William Congreve may be applied upon one centre instead of three. In the following figure, _a b c d_ represents a drum-wheel or cylinder, moving on a horizontal axis surrounded with a band of sponge 1 2 3 4 5 6 7 8, and immersed in water, so that the surface of the water touches the lower end of the cylinder. Now then, if, as in Fig. 2, the water on the descending side _b_ be allowed to accumulate in the sponge at _x_, while, on the ascending side D, the sponge at the water's edge shall, by any means not deranging the equilibrium, be so compressed that it shall quit the water in a dry state, the accumulation of water above its level at _x_, by the capillary attraction, will be a source of constant rotary motion; and, in the present case, it will be found that the means of compressing the sponge may be best obtained by buoyancy, instead of weight. For this purpose, therefore, the band of sponge is supposed to be divided into eight or more equal parts, 1 2 3 4, etc., each part being furnished with a float or buoyant vessel, _f_ 1, _f_ 2, etc., rising and falling upon spindles, _s s s_, etc., fixed in the periphery of the drum; these floats being of such dimensions that, when immersed in water, the buoyancy or pressure upwards of each shall be sufficient to compress that portion of the sponge connected with it, so as to squeeze out any water it may have absorbed. These floats are further arranged by means of levers _l l l_, etc., and plates _p p p_, etc., so that, when the float _f_ No. 1 becomes immersed in the water, its buoyant pressure upwards acts not against the portion of the sponge No. 1, immediately above it, but against No. 2, next in front of it; and so, in like manner, the buoyancy of _f_ No. 2 float acts on the portion of the sponge No. 3, and _f_ No. 3 float upon No. 4 sponge.