In the Allen engine,[85] the cylinder and frame are connected as in the engine seen above, and the crank-disk, shaft-bearings, and other principal details, are not essentially different. The valve-gear[86] differs in having four valves, one at each end on the steam as well as on the exhaust side, all of which are balanced and work with very little resistance. These valves are not detachable, but are driven by a link attached to and moved by an eccentric on the main shaft, the position of the valve-rod attachment to which link is determined by the governor, and the degree of expansion is thus adjusted to the work of the engine. The engine has usually a short stroke, not exceeding twice the diameter of cylinder, and is driven at very high speed, generally averaging from 600 to 800 feet per minute.[87] This high piston-speed and short stroke give very great velocity of rotation. The effect is, therefore, to produce an exceptional smoothness of motion, while permitting the use of small fly-wheels. Its short stroke enables entire solidity to be attained in a bed of rigid form, making it a very completely self-contained engine, adapted to the heaviest work, and requiring only a small foundation. [85] The invention of Messrs. Charles T. Porter and John F. Allen. [86] Invented by Mr. John F. Allen. [87] Or not far from 600 times the cube root of the length of stroke, measured in feet. The journals of the shaft, and all cylindrical wearing surfaces, are finished by grinding in a manner that leaves them perfectly round. The crank-pin and cross-head pin are hardened before being ground. The joints of the valve-gear consist of pins turning in solid ferrules in the rod-ends, both hardened and ground. After years of constant use thus, no wear occasioning lost time in the valve-movements has been detected. High speed and short strokes are essential elements of economy. It is now well understood that all the surfaces with which the steam comes in contact condense it. Obviously, one way to diminish this loss is to reduce the extent of surface to which the steam is exposed. In engines of high speed and short stroke, the surfaces with which the steam comes in contact, while doing a given amount of work, present less area than in ordinary engines running at low speed. Where great steadiness of motion is desired, the expense of coupled engines is often incurred. Quick-running engines do not require to be coupled; a single engine may give greater uniformity of motion than is usually obtained with coupled engines at ordinary speeds. The ports and valve-movements, the weight of the reciprocating parts, and the size and weight of the fly-wheels, should be calculated expressly for the speeds chosen. The economy of the engine here described is unexcelled by the best of the more familiar "drop cut-off" engines.