STEAM IN THE CYLINDER. 183
aperture in a thin plate of the same sectional area, owing to the bends and lateral friction of the long passage. It therefore frequently happens that the opening of the port allowed by the valve, though it may be much less than the total area of the port, is sufficiently large to pass all the steam that can force its way along the passage. This fact is constantly ex- emplified in practice: it is known that the opening of the port beyond a certain amount, which is in all cases less than the area of the port itself, ceases to be advan- tageous in facilitating the passage of the steam into the cylinder. Similarly, the opening of the regulator, or "throttle-valve," beyond a small fraction of the sectional area of the steam-pipe, does not add to the available pressure in the valve-cheat. When the steam is not dry, containing water in suspension, the labour of moving in passages is greatly increased, owing to the quantity of dead, inelastic weight to be dragged along; and the reduction of pressure is consequently much more than with dry steam. Directing attention, for the present, to the behaviour of steam within the cylinder, it is to be premised that, notwithstanding the objection that has been urged against the ordinary slide-valve, worked by an excentric motion--the want of sufficient celerity of action-there is no material wire-drawing of the steam by the closing valve when the period of admission exceeds two-thirds of the stroke, unless at very high speeds of piston, exceeding from 500 to 600 feet per minute. When the steam is cut off at shorter periods, however, the travel of the valve being less, and therefore, also, its velocity of motion, the wire-drawing increases at high speeds, though at low speeds it does not. For example,
