At about the same time he built the now standard form of return tubular boilers for moderate pressures. In the figure, _S_ is the steam and _W_ the water space, and _F_ the furnace. The direction of the currents of smoke and gas are shown by the arrows. [Illustration: FIG. 89.--Stevens's Return Tubular Boiler, 1832.] Some years later (1840), Stevens commenced using steam-packed pistons on the Trenton, in which steam was admitted by self-adjusting valves behind the metallic packing-rings, setting them out more effectively than did the steel springs then (and still) usually employed. His pistons, thus fitted, worked well for many years. A set of the small brass check-valves used in a piston of this kind, built by Stevens, and preserved in the cabinets of the Stevens Institute of Technology, are good evidence of the ingenuity and excellent workmanship which distinguished the machinery constructed under the direction of this great engineer. [Illustration: FIG. 90.--Stevens's Valve-Motion.] The now familiar "Stevens cut-off," a peculiar device for securing the expansion of steam in the steam-cylinder, was the invention (1841) of Robert L. Stevens and a nephew, who inherited the same constructive talent which distinguished the first of these great men--Mr. Francis B. Stevens. In this form of valve-gear, the steam and exhaust valves are independently worked by separate eccentrics, the latter being set in the usual manner, opening and closing the exhaust-passages just before the crank passes its centre. The steam-eccentric is so placed that the steam-valve is opened as usual, but closed when but about one-half the stroke has been made. This result is accomplished by giving the eccentric a greater throw than is required by the motion of the valve, and permitting it to move through a portion of its path without moving the valve. Thus, in Fig. 90, if _A B_ be the direction of motion of the eccentric-rod, the valve would ordinarily open the steam-port when the eccentric assumes the position _O C_, closing when the eccentric has passed around to _O D_. With the Stevens valve-gear, the valve is opened when the eccentric reaches _O E_, and closes when it arrives at _O F_. The steam-valve of the opposite end of the cylinder is open while the eccentric is moving from _O M_ to _O K_. Between _K_ and _E_, and between _F_ and _M_, both valves are seated. _H B_ is proportional to the lift of the valve, and _O H_ to the motion of the valve-gear when out of contact with the valve-lifters. While the crank is moving through an arc, _E F_, steam is entering the cylinder; from _F_ to _M_ the steam is expanding. At _M_ the stroke is completed, and the other steam-valve opens. The ratio (E M)/(E L) is the ratio of expansion.