The compression line should be a hyperbolic curve, beginning with the period of compression. In calculating both the compression and expansion, allowance must be made for the clearance space and steam-way. In a cylinder like that illustrated in fig. 92, their contents would be about equal to that of two inches of the cylinder. Therefore, when steam is cut off at 8 in. of the stroke, instead of having a quantity of steam which will fill a cylinder 16 in. diameter and 8 in. long, we have as much as would fill a cylinder of that diameter and 10 in. long. The same thing is true of the compression. This must occur in the above example when the piston has 6¹⁄₂ in. more to move before completing its stroke. There is therefore a quantity of steam in front of it sufficient to fill a cylinder 8¹⁄₂ in. in diameter. This steam is of course compressed by the advance of the piston, and if its pressure when compression begins is the same as that of the atmosphere, then it will be 0.9 lbs. above it when the piston has only 6 in. to move and 3.2, 6.2, 10.5, 16.9, and 27.5 lbs. effective pressure when the piston has 5, 4, 3, 2 and 1 inches to move respectively, and when pre-admission begins, the pressure will have risen to 48.7 lbs. If the back pressure is above that of the atmosphere, of course the compression will be correspondingly increased. It will also be seen that, without any or with very little clearance space, the compression would at the end of each stroke rise above the boiler pressure. It being a peculiarity of the ordinary shifting-link motion that as the period of admission is reduced that of compression is lengthened, the latter becomes very excessive when the steam is cut off at less than one-third or one-fourth of the stroke. QUESTION 217. _In what respect would a diagram made by an indicator differ from the theoretical form represented in fig. 136b?_ _Answer._ It would be drawn with less exactness; that is, the corners instead of being sharply defined, as in fig. 136b, would be more or less rounded, as in fig. 137, and the curves and straight lines would vary somewhat from the exact mathematical form indicated in fig. 136b. The higher the speed at which the engine is working when the diagrams are taken, the greater will be the variation from the theoretical form. [Illustration: _Fig. 137._] QUESTION 218. _If the amount of pre-admission is insufficient, how will it be shown in the indicator diagram?_