“In employing a high grade of expansion, especially with the considerable pressure of steam now usually carried in stationary boilers, two serious practical difficulties are met with. The first arises from the injurious effect of the sudden application of so great a force on the centers, which the beam-engine, indeed, cannot be made to endure, and the second is found in the extreme difference between the pressures at the opposite ends of the stroke, which is such that the crank, instead of being acted upon by a tolerably uniform force, is rotated by a succession of violent punches, and these applied when it is in its most unfavorable position.... “In the Allen engine the action of high speed causes all the practical difficulties which lie in the way of the successful employment of high grades of expansion combined with high pressure of steam completely to disappear. The crank receives as little pressure on the centers as we please; none at all if we like; the force is applied to it as it advances, in a manner more gradual than the advocates of graduated openings and late admission ever dreamed of, and a fair approximation is made to a uniform rotative force through the stroke. So that, in a properly constructed engine, the higher the speed the smoother and more uniform and more silent the running will be.” After a page or more devoted to a demonstration of this action, Mr. Colburn sums up the advantage of high speed in the following illustration: “Let us suppose that, in an engine making 75 revolutions per minute, the reciprocating parts are of such a weight that the force required at the commencement of the stroke to put them in motion is equal to a pressure of 20 pounds on the square inch of piston. This will not modify the diagram of pressure sufficiently to produce much practical effect. But let the number of revolutions be increased to 150 per minute, the centrifugal force of these parts as the crank passes the centers is now equal to 80 pounds on the square inch of piston, and any pressure of steam below this amount acts only as a relieving force, taking the strain of these parts partly off from the crank. It makes no matter how suddenly it is admitted to the cylinder, not an ounce can reach the crank; but as the latter advances, and the acceleration of the reciprocating parts becomes less, the excess of force not required to produce this becomes, in the most gradual manner, effective on the crank. “It will be observed how completely the designer has this action of the reciprocating parts under control. He can proportion their speed and weight to the pressure of steam in such a manner as to relieve the crank from the blow on the center to whatever extent he may wish. The notion that the reciprocating parts of high-speed engines should be very light is therefore entirely wrong. They should be as heavy as they can be made, and the heavier the better.