[Illustration] When steam of great elasticity is required, the weight is placed at the extremity of the lever, and as such, acts with greater force on the safety-valve, than when removed to a point nearer to the axis on which it revolves: so that should _low_ pressure steam, or that which has a less expansive force, be required, it will only be necessary to remove it nearer towards the axis on which it turns. The lever and balance-ball safety valve already described, appear but little calculated for those engines in which high pressure steam is employed, as the engine-man, in an over anxious zeal for the full performance of the machinery confided to his care, has been frequently known to increase the internal pressure of a large boiler many thousand pounds beyond the resistance to which it was originally proved. To prevent a recurrence of those accidents, which first drew the attention of the legislature to this important part of the engine, it appears advisable to inclose the safety-valve in an iron case, of which a section is annexed. [Illustration] The valve _B_ in this case rests upon a conical seat in the boiler _A_, and is furnished with a series of small moveable plates lettered _c_, which are employed to increase or diminish the entire weight of the safety-valve, the whole being covered by the box _D_; and as this is pierced with a number of small holes, the steam readily escapes when the expansive force exceeds the resistance offered by the loaded valve. The patent _revolving wheel_ invented by Mr. Masterman, appears to promise the best results of any rotatory engine yet invented, the friction being much less than in any other apparatus in which steam is employed as a prime mover. In this engine, Mr. Masterman proposes to employ water, or the fluid metal mercury as the immediate agent, which he effects by inclosing it in the tubular rim of a large wheel, furnished with valves opening in one direction. This wheel, as is shewn in the opposite diagram, is made to revolve on a hollow axis connected with the steam boiler. The arms or spokes which radiate from the axis are also hollow; and on the admission of steam from the boiler, it is conducted through the arm immediately opposite, and entering the rim of the wheel, comes in contact with, and presses against the column of water beneath and the closed valve above the arm. The water being previously heated to the boiling point, no condensation ensues, but the whole weight of water, which was previously balanced in two columns of equal height, is driven, by the pressure of the steam, to the side opposite to that at which the elastic vapour entered, and that side of the wheel will necessarily preponderate. If this process be repeated, the steam being allowed to blow through each radiating arm in succession, a continuous rotatory motion will be produced. Should it be advisable to employ steam of less elasticity, a condenser may be added, and