towards the axle, thus causing an increase of lead on the opposite side of the valve. We have employed for our illustrations very short eccentric rods, in order to make this action apparent by exaggerating it. It is obvious from the engravings that the difference in the lead is increased as the eccentric rods are shortened, and also as the distance between the points of connection of the rods with the link is increased. It will also be plain that increasing the throw of the eccentrics, that is, increasing the distance of the centres _s_, _s_, of the eccentrics from the centre _S_ of the axle will also increase the variation in the lead in full and mid-gear. QUESTION 201. _What is meant by the distribution of steam in the cylinder?_ _Answer._ It means the admission and exhaust of steam to and from the cylinder in relation to the stroke of the piston or the revolution of the crank. QUESTION 202. _What are the principal periods or elements of the distribution of steam by the slide-valve and link motion?_ _Answer._ They are: 1. The _pre-admission_ or lead, that is, the admission of steam into the cylinders in front of the piston before it has completed its stroke. 2. The _admission_ of steam after the piston has commenced its stroke. 3. The _expansion_ of steam in the cylinder. 4. The _pre-release_, or exhaust of steam before the piston has completed its stroke. 5. The _release_, or exhaust during the return stroke of the piston. 6. The _compression_ of steam, or closing the exhaust before the piston has completed its return stroke. QUESTION 203. _What is meant by the clearance of the piston?_ _Answer._ It is the space between the piston and the cylinder-head when the former is at the end of the stroke. If the piston touched the cylinder-head at the end of each stroke, it would cause a concussion or “thump” which would injure these parts. Owing to the impossibility of constructing machinery with absolute accuracy, it is therefore necessary to leave a space, usually from ¹⁄₂ to ¹⁄₄ in. wide, between the piston and the cylinder-heads, so as to be certain that they will not strike each other should there be any slight inaccuracies in the length of the piston-rods, connecting-rods, frames or other parts. QUESTION 204. _Why is it desirable to open the steam-port and admit steam at the end of the cylinder towards which the piston is moving BEFORE the latter has completed its stroke?_