1. Because when steam of a high pressure is introduced into the cylinder, and allowed to expand until its pressure is comparatively low, it escapes at a lower pressure than the average pressure during the whole stroke. If steam of a pressure equal to the _average_ pressure is worked full stroke, it would exert exactly the same force on the piston as the steam of higher pressure did when working expansively, but the pressure in the latter case, when the piston reaches the end of the stroke, or the _final pressure_, as it is called, would be considerably lower than in the other. The pressure of steam represents _energy_, or _capacity for doing work_, and therefore if we allow it to escape with a comparatively high pressure without doing work, it is a waste of energy. To illustrate this, we will take the same conditions which were used in the answer to Question 57, in calculating the average pressure. In that case the mean absolute pressure of the steam was 69.95 pounds per square inch, but the pressure at the end of the stroke, when the steam escaped, was only 33²⁄₃ pounds absolute. If, therefore, steam had been used of the average pressure through the whole stroke, it would have escaped with a pressure of 69.95 pounds, or more than twice that of the expanded steam, and the work done in both cases would have been the same. 2. There is also another incidental advantage in this, because low-pressure steam can be exhausted more quickly from a cylinder than steam of a high pressure, and consequently there is less resistance, or _back pressure_, as it is called, in the exhausted end of the cylinder to the movement of the piston. 3. The causes which produce the greatest economy when steam is used expansively cannot be fully explained without discussing principles of science more abstruse than it is desirable to introduce here. They can, however, with the aid of the table of the “Properties of Steam,”[18] in the appendix, be illustrated by a few simple calculations, so that the economy of using steam expansively will be apparent. [18] This table is copied from Colburn’s Treatise on the Locomotive Engine. For the basis of the calculations the same data and dimensions will be employed that were used in the previous illustration; that is, a cylinder of 16 in. diameter and piston with 24 in. stroke and steam of 100 lbs. absolute pressure cut off at 8 in. of the stroke. We will suppose, further, that the steam used is generated from water of a temperature of 60 degrees, and we will then calculate the total number of units of heat in the steam used for each stroke of the piston. The area of a piston 16 in. in diameter is 201 square inches; and as the steam is admitted until the piston moves 8 inches of its stroke, therefore the quantity of steam would be 8 times 201 cubic inches, or 1608 201 × 8 = 1608 cubic in. = ---- cubic ft. 1728