off the steam at three-fourths of the stroke of the piston. Now, if the eccentric rod of one of these eccentrics be attached to the top end of a long bar 'y', and the rod of the other eccentric be attached to the bottom end of that bar, the top and bottom ends of that bar will have the same relative position to the right and to the left as the centres of the two eccentrics have, that is, as the line a y, uniting those points, has (although the bar may not be parallel to the line a y, because it is much longer than that line). If, therefore, the slide rod be attached to any point in this bar, the rod will have precisely the same motion as if it were driven by a crank pin placed in the same relative position in the line x y that the point of attachment is placed in, in the bar x'y'. For example, if it be placed at the top end of the bar, it will have the exact motion which would be given by the eccentric which is in connection with the top end; if at the bottom end of the bar, then it would have the exact motion which would be given by the eccentric in connection with that end; if at the middle of the bar, then it would have the motion that would be given to the centre of the line xy, which, you will remember under the circumstances stated, is one which would just, not open the steam passage at all.

We will now put the model into operation, and show that when the crank is, say, at its left hand (see diagram 39A, fig. A), the link will be vertical, and therefore the moving of the slide stalk up and down this link will have no effect whatever in changing the position of the slide valve. Similarly, when the crank is turned half round so as to bring the crank to the right-hand end, the link will again be vertical, and again there will be no effect by altering the point where the slide stalk engages with the link. But let us next put the crank to an angle of 60 degs.