QUESTION 61. _What is the economical effect of reducing the pressure, or of wire-drawing it, by partly closing the valve by which it is admitted to the cylinders?_ _Answer._ By reducing the pressure of steam in this or any other way, it is necessary in doing the same amount of work to admit steam to the cylinder for a longer period, and therefore to reduce the degree of expansion. To illustrate the effect of this, we will estimate the total heat required to exert a pressure of 70 lbs. on the piston described above. It will be assumed that the steam pressure in the boiler is 100 lbs. absolute, and that this is wire-drawn down to 70 lbs. and admitted to the cylinder through the whole stroke. As was shown in the preceding answer, 4,824 cubic inches of steam are required to fill the cylinder. Now 3,376.8 cubic inches of steam of 100 lbs. pressure, if expanded to 70 lbs. pressure, will make 4,824 cubic inches. The total heat required to generate 3,376.8 cubic inches of steam of 100 lbs. absolute pressure from water of 60 degrees is 519.9 units, so that to do the same work by using steam of _high pressure cut off at one-third of the stroke_, using steam of _low boiler pressure full stroke_, and using _wire-drawn steam full stroke_, would, in the example we have selected, require 247.5, 527 and 519.9 units of heat respectively. QUESTION 62. _To what extent can we work steam expansively, with advantage and economy?_ _Answer._ The theoretical economy of using steam increases with the degree of expansion and the pressure. This is shown very clearly in the following table, in the first column of which the number of inches of the piston stroke is given during which steam is admitted to a cylinder 16 in. in diameter and 24 in. stroke. In the second column is given the pressure of the steam, or _initial pressure_, as it is called, which must be admitted into the cylinder in order to produce a mean pressure of 70 lbs. per square inch when it is cut off at the point indicated in the first column. In the third column is given the total heat which is required to generate the steam required in each case, and in the last column the percentage of saving is given which results from the different degrees of expansion and a mean pressure of 70 lbs. per square inch in each case. RESULTS OF USING STEAM EXPANSIVELY.