In Fig. 24 we see in section the slide-valve, the ports of the cylinder, and part of the piston. To the right are two lines at right angles--the thicker, C, representing the position of the crank; the thinner, E, that of the eccentric. (The position of an eccentric is denoted diagrammatically by a line drawn from the centre of the crank shaft through the centre of the sheave.) The edges of the valve are in this case only broad enough to just cover the ports--that is, they have no _lap_. The piston is about to commence its stroke towards the left; and the eccentric, which is set at an angle of 90 deg. in _advance_ of the crank, is about to begin opening the left-hand port. By the time that C has got to the position originally occupied by E, E will be horizontal (Fig. 25)--that is, the eccentric will have finished its stroke towards the left; and while C passes through the next right angle the valve will be closing the left port, which will cease to admit steam when the piston has come to the end of its travel. The operation is repeated on the right-hand side while the piston returns. [Illustration: FIG. 26.] It must be noticed here--(1) that steam is admitted at full pressure _all through_ the stroke; (2) that admission begins and ends simultaneously with the stroke. Now, in actual practice it is necessary to admit steam before the piston has ended its travel, so as to _cushion_ the violence of the sudden change of direction of the piston, its rod, and other moving parts. To effect this, the eccentric is set more than 90 deg. in advance--that is, more than what the engineers call _square_. Fig. 26 shows such an arrangement. The angle between E and E^1 is called the _angle of advance_. Referring to the valve, you will see that it has opened an appreciable amount, though the piston has not yet started on its rightwards journey. "LAP" OF THE VALVE--EXPANSION OF STEAM.