at comparatively slow speeds; but if it is thought desirable, the period of admission and the point of release for both strokes can be equalized, either by giving the valve more lead or lap at one end than the other, or by making the one steam-port wider than the other. The mechanism employed for moving locomotive slide-valves, however, furnishes us with the means of modifying their motion in relation to that of the piston, and of thus equalizing the periods of admission and release for the front and back strokes. The methods of doing this will be more fully explained hereafter. PART V. THE EXPANSION OF STEAM. [Illustration: _Fig. 30._ Scale ²⁄₃ in. = 1 foot.] [Illustration: _Fig. 31._ Scale ¹⁄₄ in. = 1 inch.] QUESTION 55. _How can we determine by experiment the pressure of the steam in the cylinder at all points of the stroke?_ _Answer._ By the use of an instrument made for that purpose, called an _indicator_. Its action can be best explained by supposing that we have a small cylinder and piston, _T_, fig. 30, (shown on an enlarged scale in fig. 31) attached by a pipe _U_ to one end of the cylinder _A_, so that when steam is admitted to the latter it will be conducted to the small cylinder _T_ through the pipe _U_. Over the small piston and attached to it is a spiral spring, _s s_, fig. 31, which is compressed when the piston rises and extended when it falls. To the top of the piston-rod, _V_, a pencil, _W_, is attached. Behind this pencil we will suppose there is a card, _a b c d_, and that this card is so arranged that we can slide it horizontally and in contact with the pencil point. With only the pressure of the atmosphere above and below the piston _T_, the spring would be neither compressed nor extended, and the piston would then stand in the position shown in fig. 31. If now we move card horizontally, the pencil will draw a line, _g_, _h_, called the _atmospheric line_. We will now suppose that the tension of the spring is such that a pressure of 10 lbs. per square inch above or below the piston will either extend or compress the spring ¹⁄₄ inch. In other words, every pound of pressure per square inch in the piston will move it ¹⁄₄₀ of an inch. If we could produce a vacuum under the piston, it would be pressed down by the atmosphere above it ¹⁵⁄₄₀, or ³⁄₈ of an inch. If, when it is thus depressed, we again slide the card along in contact with the pencil-point, it will draw another line, _e_, _f_, called the _vacuum-line_. Assuming that we have drawn these two lines, and that the piston and card are in the position shown in figs. 30 and 31, we will then suppose that a reciprocating motion can be given to the card by the lever _L_, _M_, _N_, fig. 30, which is pivoted at _M_ and attached at _N_ to the piston-rod by a short connecting-rod. It is obvious that by connecting the upper end _L_ of the lever with a rod, _L c_, to the card, the latter will be moved backwards and forwards