The perfection of a steam valve, other things being equal, consists in the degree of nicety with which its motion is timed relatively to the motion of the piston.

The functions of the piston are absolutely dependent upon the proper timing of the admission and release of the steam. A very slight and apparently trifling error in the adjustment produces a most serious effect upon the consumption of fuel. If from any cause the valve should open to admit steam for a fresh stroke before the preceeding stroke is finished, it opens too soon, and an unnecessary resistance to the piston is produced.

If on the other hand, the valve should delay its opening until the piston had begun its return, it opens too late, because thus the steam has to fill uselessly the space left vacant, and hence a waste of steam, and a loss of power.

As far then as the admission of steam is concerned, it is a necessary condition that the steam ports should open neither before nor after, but at the precise moment when the stroke commences. Some engineers recommend giving the valve "lead" as it is termed, that is to say, setting it so as to open a little before the end of the stroke, but it is an open question whether the slightest advantage is gained by so doing to a greater extent than is necessary to compensate for any slackness or lost motion in the valve.

gearing, or for their expansion when heated by the steam, and of an inch is quite sufficient in 1% a well constructed engine.

It is also an open question whether it would not be better to bring the piston to a state of rest by the "compression" of the exhaust steam than by means of any lead to the steam valve at all.

Now the valve shown in Fig. 19, ratifies the conditions for the admission of the steam: it opens exactly at the right time, the steam begins to enter as the piston begins to move, as it foliows it steadily and effectively throughout the stroke. Whatever time the piston takes for its journey, the steam is allowed as much time to follow it. At first the opening is small, but then the motion of the piston is comparatively slow, and therefore the supply keeps pace with the demand.

As respects the release of the steam when the stroke has been completed, the performance of this valve is altogether unsatisfactory, and here lurks the cause of the difference in the performance of the old and the later engines. But it may be said that the release does appear to take place at the right time, because it occurs just when the piston has finished its stroke, and if it were to occur before, a loss of power would

ensue.

This is a very plausible view of the case, and

the one which delayed for years the saving of fuel which has since been effected.

Sufficient attention was not bestowed upon what was going on in the cylinder, or upon the facts which might have indicated them; to fill and empty a cylinder full of steam are operations requiring time.

The time required for filling the cylinder with steam necessarily corresponds with the duration of the stroke, whatever its duration may be. But this cannot be the case as regards the second operation-emptying of the cylinder.

This ought to be performed in an instant, or otherwise the steam continues pent up when it ought to be liberated, when it ought to assume its minimum pressure-the pressure of the atmosphere-and exerts an injurious counter-pressure against the piston, tending to increase the resistance to be overcome. To effect the free and rapid discharge it is necessary not merely to open the communication to the exhaust pipe, but to open a wide passage and to have this done by the time the piston commences the return stroke.

The valve alluded to cannot accomplish this, its motion being gradual, not instantaneous, as it should be.

The passage only begins to open when the piston is in turn, and it is not wide open until the piston has travelled through one-tenth of its

entire stroke. The steam in the cylinder is re strained from escaping, being, as it were, wiredrawn in the passage out, and consequently takes considerable time to assume the pressure of the atmosphere. In the meanwhile the new stroke has begun and been partly completed, and so far the piston has had to contend with a resistance altogether illegitimate, a resistence in many cases-especially at high speeds nearly equal to all the other resistances put together. It was not until the year 1838 that the true cause of the trouble was suspected, and a remedy applied.

It had been thought before that time, that giving an engine lead tended to improve its speed when already running at a high rate.

This was attributed to the opening of the steam port being wide at the commencement of the stroke, thereby increasing the facility for the entrance of the steam in following up the piston.

Its true explanation was found to be the earlier release of the waste steam and consequent diminution of resistance. As sometimes threeeighths of an inch, or even one-half of an inch "lead" was given to high-speed engines, it was decided to try the effect of opening the exhaust passage earlier by the same amount, while the steam port should still be made to open only at the beginning of the stroke.

An engine was chosen for the experiment the valve of which resembled Fig. 20, placing the

valve on the ports so as to allow the exhaust passage to be three-eighths of an inch open, the steam port at the same time to be one-quarter of an inch open. This space, therefore, was closed by adding to the length of the valve at each end one-quarter of an inch.

The eccentric was, of course, shifted on the shaft to correspond with the alteration, and the engine with the altered valve (see Fig. 20) again set to work.

It is almost useless to say the saving of fuel was very great. The amount by which the valve at each end overlaps the steam ports (see Fig. 20) when placed exactly over them is technically termed the "lap." The lap of the valve being three-eighths of an inch, the exhaust passage was about three-eighths of an inch open when the stroke was finished.

LAP.

The importance of putting lap upon a slide valve will be better understood by noting what would happen without it.

If there were no lap the opening for steam