The addition of inside lap has the effect of closing the port earlier than it would be closed without, and thus shortening the period of release and also of reducing the area of the opening of the port. This will be apparent by referring to fig. 128, in which the valve had ¹⁄₁₆ in. lead. The dotted lines which represent the edges of the ports in relation to the exhaust edges of the valve are therefore drawn ¹⁄₁₆ in. from the centre line _a b_. If, however, there had been no inside lap, then the edges of the ports would have conformed to the line _a b_. It will be observed that the first curve crosses the dotted line _g g′_ at 15¹⁄₂ in. of the forward stroke, which is the point at which the port is closed to the exhaust, or where the period of release ends and compression begins. If there had been no lap and the line _g g′_ had therefore occupied the same position as _a b_, then the motion-curve would not have crossed it until the piston had reached 16 in. of its stroke, thus showing that the period of release had been lengthened and compression delayed. As the width of the opening of the port is represented by the distance of the motion-curve from the right hand side of the line _g g′_, which represents the edge of the port, it is obvious that if there had been no lap, so that the position of the line representing the edge of the port had occupied the position of _a b_, then the space between it and the motion-curve would have been greater, thus showing that the port would have been opened wider if there had been no inside lap. The width of the opening of the port to the exhaust is in fact always diminished by an amount equal to the inside lap. With the same travel, increase of outside lap and lead shortens the period of release, but has no effect on the width of the opening of the port to the exhaust. Increase of travel, with the same outside lap, lengthens the period of release and also increases the width of the opening of the port to the exhaust. QUESTION 212. _What governs the period of compression?_ _Answer._ As compression begins when release ends, or when the port is closed to the exhaust, it is controlled by exactly the same causes, and as the two events occur simultaneously, of course whatever shortens the period of release lengthens that of compression. QUESTION 213. _What effect do the clearance spaces and steam-ways have upon the compression of the confined steam?_