_Answer._ BY MULTIPLYING THE ABSOLUTE PRESSURE PER SQUARE INCH OF THE STEAM IN THE CYLINDER BEFORE IT IS CUT OFF, BY THE DISTANCE FROM THE BEGINNING OF THE STROKE AT WHICH IT IS CUT OFF, AND DIVIDING THE PRODUCT BY THE WHOLE LENGTH OF THE STROKE. Thus, if we have a cylinder whose piston has a stroke of 24 inches, if we cut off the steam at 8 inches, and have an ABSOLUTE pressure of 90 lbs. in the cylinder, the calculation is as follows: 90 × 8 ------ = 30 lbs. final pressure. 24 If we cut off at 10, 12 and 15 inches, the final pressure would be 37¹⁄₂, 50 and 56¹⁄₄ lbs., respectively. To get the effective pressure deduct the atmospheric pressure from this result. QUESTION 22. _What is the proportion between the volume of steam and that of the water from which it is formed?_ _Answer._ At the pressure of the atmosphere (15 lbs.) each cubic inch of water will make 1,610 cubic inches of steam. At double that pressure, or 30 lbs. absolute pressure, it will make a little more than half as much, or 838 cubic inches; at four times, or 60 lbs. absolute pressure, 437 cubic inches, or a little more than a fourth as much as at the pressure of the atmosphere. QUESTION 23. _Why is it that the quantity of steam at high pressures is somewhat greater than in inverse proportion to the pressure?_ _Answer._ Because the boiling-point of water, as has already been explained, is higher as the pressure increases, and therefore the temperature of the steam produced at such pressure is also higher than at lower pressures; and as all gases are expanded by heat, therefore the volume of steam at the higher pressures is somewhat greater than in inverse proportion to its pressure, on account of being somewhat expanded by its high temperature. To make this plain, if we take a cubic inch of water and convert it into steam of atmospheric pressure, its volume will be 1,610 times that of the water, and its temperature 212 degrees.[6] If we convert this quantity of water into steam with a pressure double that of the atmosphere, the volume of the steam will be 838 times that of the water and its temperature will be 250.4 degrees. If the volume of the steam were exactly _inversely proportional_ to the pressure, the cubic inch of water at double the atmospheric pressure would make only 805 cubic inches of steam; but as the boiling-point at that pressure is 38.4 degrees higher, the steam is expanded 33 cubic inches by the increase of its heat due to the higher boiling-point. [6] More accurately, 213.1 degrees, if we call the atmospheric pressure 15 lbs., as we have. A table in the appendix gives the pressure, temperature and volume of saturated steam up to 300 lbs. absolute pressure. QUESTION 24. _What is meant by the condensation of steam?_