It is evident, under such circumstances, that the pressure of the steam admitted from the boiler must be much greater than the resistance opposed to the piston, and that the motion of the piston must, in the first instance, be accelerated and not uniform. If the piston moved from the commencement with a uniform motion, the pressure of the steam urging it must necessarily be exactly equal to the resistance opposed to it, and then cutting off the supply of steam from the boiler, the piston could only continue its motion by inertia, the steam immediately becoming of less pressure than the resistance; and after advancing through a very small space, the piston would recoil upon the steam, and come to a state of rest. The steam, however, at the moment it is cut off being of much greater pressure than the amount of resistance upon the piston, will continue to drive the piston forward, until by its expansion its force is so far diminished as to become equal to the resistance of the piston. From that point the impelling power of the steam will cease, and the piston will move forward by its inertia only. The point at which the steam is cut off should therefore be so regulated that it shall acquire a pressure equal to the resistance on the piston by its expansion, just at such a distance from the end of the stroke as the piston may be able to move through by its inertia. It is evident the adjustment of this will require great care and nicety of management. (79.) In 1797 a patent was granted to the _Rev. Mr. Cartwright_, a gentleman well known for other mechanical inventions, for some improvements in the steam-engine. His contrivance is at once so elegant and simple, that, although it has not been carried into practice, we cannot here pass it over without notice. The steam-pipe from the boiler is represented cut off at B (fig. 46.); T is a spindle-valve for admitting steam above the piston, and R is a spindle-valve in the piston; D is a curved pipe forming a communication between the cylinder and the condenser, which is of very peculiar construction. Cartwright proposed effecting a condensation without a jet, by exposing the steam to contact with a very large quantity of cold surface. For this purpose, he formed his condenser by placing two cylinders nearly equal in size one within the other, allowing the water of the cold cistern in which they were placed to flow through the inner cylinder, and to surround the outer one. Thus the thin space between the two cylinders formed the condenser. [Illustration: Pl. IX.] [Illustration: Pl. X.]