The wheel, which is represented in the centre of the engraving, is exactly similar to a common water wheel; the treadboards upon its circumference are, however, of considerable length, so as to allow sufficient standing room for a row of from ten to twenty persons upon the wheel. Their weight, the first moving power of the machine, produces the greatest effect when applied upon the circumference of the wheel at or near the level of its axle; to secure therefore this mechanical advantage, a screen of boards is fixed up in an inclined position above the wood, in order to prevent the prisoners from climbing or stepping up higher than the level required. A hand rail is fixed upon this screen, by holding which they retain their upright position upon the revolving wheel, the nearest side of which is exposed to view in the plate, in order to represent its cylindrical form much more distinctly than could otherwise have been done. In the original, however, both sides are closely boarded up, so that the prisoners have no access to the interior of the wheel, and all risk of injury whatever is prevented. By means of steps the gang of prisoners ascend at one end, and when the requisite number range themselves upon the wheel, it commences its revolutions. The effort, then, to every individual is simply that of ascending an endless flight of steps, their combined weight acting upon every successive stepping board precisely as a stream of water upon the float boards of a water wheel. During this operation each prisoner gradually advances from the end at which he mounted towards the opposite end of the wheel, from whence the last man taking his turn descends for rest, another prisoner immediately mounting as before to fill up the number required, without stopping the machine. The interval of rest may then be portioned to each man by regulating the number of those required to work the wheel with the whole number of the gang; thus if twenty-four are obliged to be upon the wheel, it will give to each man intervals of rest amounting to twelve minutes in every hour of labor. Again, by varying the number of men upon the wheel, or the work inside the mill, so as to increase or diminish its velocity, the degree of hard labor or exercise for the prisoners may also be regulated. At Brixton, the diameter of the wheel being five feet, and revolving twice in a minute, the space stepped over by each man is 2193 feet. From the _Salem Register_. -------------------------