But with steam of a given pressure, the velocity of the piston will be different: with a given capacity of cylinder, and a given pressure of steam, the power of the engine will determine the number of strokes per minute. But the actual velocity of the piston will depend, in that case, on the proportion which the diameter of the cylinder bears to its length; the greater the diameter of the piston is with respect to its length, the less will be its velocity. In case of stationary engines used on land, that proportion of the diameter of the cylinder to its length is selected, which is thought to contribute to the most efficient performance of the machine. According to some engineers, the length of the cylinder should be twice its diameter;[52] others make the length equal to two diameters and a half; but there are circumstances in which considerations of practical convenience render it necessary to depart from these proportions. In marine engines, where great length of cylinder would be inadmissible, and where, on the other hand, considerable power is required, cylinders of short stroke and great diameter are used. In these engines the length of the stroke is often not greater than the diameter of the piston, and sometimes even less. [Footnote 52: This is the proportion under which the cylinder with a given capacity will present the least possible surface to the cooling effect of the atmosphere.] The actual velocity which has been found to have the best practical effect, for the piston in low-pressure engines, is about 200 feet per minute. This, however, is subject to some variation.