plains the point determined by practice as the limit of the narrowing of the orifice.

ARTICLE III.

OF THE LEAD OF THE SLIDE.

§ 1. Nature and Effects of the Lead.

The third disposition which we have to discuss, is the lead of the slide.

In describing the different parts of the engine, we said that it is the slide that opens and shuts successively the passages above and below the piston, so as to apply the effort of the steam alternately on one side and on the other. If the engine were regulated, as it appears natural that it should be, the slide would keep the passage open to the steam until the piston had reached the bottom of the cylinder. At that instant the change would take place. The first passage would be shut, and the opposite passage opened. Then the motion of the slide would accompany exactly that of the piston. Their alternation would be strictly simul

taneous.

But this is not the case; it has been found by experience, that the engine is capable of acquiring a greater speed when the motion of the slide pre

cedes a little that of the piston; that is to say, when it opens the passages to the steam a little before the necessary moment. When the engine is regulated in that manner, at the moment the piston is going to begin a new stroke, the passages, instead of beginning to open, have already a certain degree of aperture. This premature degree of aperture is called the lead of the slide, because it indicates in how far the motion of the slide precedes that of the piston. In fact, we can conceive, that if the return of the slide is, for instance, a quarter of an inch in advance on that of the piston, the passages for the steam will have a quarter of an inch aperture when the piston touches the bottom of the cylinder.

The effect of that disposition, first on the speed and then on the load, are the two points we intend to examine here.

The common way of explaining the increase of speed the engine acquires when it has a little lead, is by saying, that by that means the steam is ready to act on the piston at the moment the piston begins its stroke. But it is not difficult to see, that if the steam really acts quicker at the beginning of the stroke, it is also sooner interrupted at the end of the stroke. The effect would thus only be, to add on one side what is substracted on the other. That explanation is, therefore, by no means satisfactory.

But the manner in which the calculation of the

speed of the engines has been established here-above, gives us immediately the real explanation of the fact.

If the change in the passages of the steam, instead of occurring exactly at the end of the stroke of the piston, takes place, according to our supposition, at the moment the piston is still an inch from the bottom, from that instant no more steam enters the cylinder. In fact, on one side the passage is shut; it is true that it is open on the other, but the piston, which must necessarily finish its stroke, keeps the steam pressed back in the passages, from whence it cannot get out until the piston begins to take its retrograde direction. Thus, in regard to the quantity of steam admitted in the cylinders at each stroke of the piston, the length of that stroke is in reality diminished by an inch. We have seen that, to know the velocity of the piston, we must divide the mass of steam generated in the boiler by the area of the cylinders (Chap. V. Art. V. § 1), and that the quotient will be the speed with which that volume of steam must necessarily pass through the cylinders, or the velocity of the piston. That will really give the velocity wanted, if the steam issues without any interruption; but if, as it is here the case, there occurs at each stroke a suspension in the issuing of the steam, it is evident that, for the same quantity of steam to go through the cylinders, a greater velocity of motion will be required. It is the generation of steam in the boiler that regulates and limits the speed; if, there

fore, we suppose that the generation supplied m cylinders full of steam in a minute, when the total length of the cylinder got filled with steam, now that the length - only gets filled, the same quantity of steam will fill per minute a number of

cylinders expressed by m × —¿. Then the speed of

the piston will be augmented in the inverse proportion of the length of cylinders that get full of

steam.

We see why the lead is favourable to the speed. But if there be profit in that respect, there is loss in regard to the load that the engine is able to draw.

Suppose the line ED (fig. 25) represents the stroke of the piston, and that the stroke takes place in the direction of the arrow. The passage being shut on one side of the piston a little before it is opened on the other side, as we shall see below, let A be the point where the piston is, when the arrival of the steam is intercepted on the side E, and let C be the point where it is when the slide begins to admit the steam on the opposite side, that is to say, on the side D.

It is clear, that at the instant the piston reaches the point A, the moving power that produced the motion is suppressed. Moreover, when the piston, continuing its stroke by virtue of its acquired velocity, reaches the point C, not only has it ceased receiving any impulsion in the direction of the mo

tion, but it suffers even an opposition from the steam admitted in a contrary direction. The piston, however, cannot stop. It must finish its stroke. It must, therefore, repulse that fresh steam that opposes it. As it necessarily spends in the conflict a force equal to that which the steam would have communicated to it, the consequence is, that during the space CD there is not only suspension of the action of the moving force, but even introduction of that moving force in a contrary direction, and in the same proportion destruction of the force previously acquired.

We see, therefore, that the effect of the moving power, in regard to the motion, is only produced on the length of the stroke, first diminished of AD, and then of CD; so that, if those two distances are represented by 6 and a, the effect we are really entitled to expect from the engine is only in proportion of a stroke -- a. 7.

Now we have seen (Chap. V. Art. V. § 4) that the limit of load an engine can draw, is determined by calculating the pressure on the piston as equal to the pressure in the boiler, or expressed by

expression in which represents the stroke of the piston. It is then clear, that the limit of load will be smaller in proportion as the stroke is dimi