in all circumstances, the atmospheric pressure; and that it is only in those cases in which the speed is not taken into account that that simplification can take place without mistake.

If we sometimes find calculations of the power of locomotive engines, or any other sort of steamengines, in which there appears what is termed lost power; that is to say, calculations according to which it would appear that these engines produce in practice only one-third or even a quarter of what is termed their theoretical power; and if that difference between practice and theory be at present so generally established, that it is taken as a rule to say that practical horses are only the third part of theoretical horses, the reason is, simply, that this supposed theoretical power is wrongly calculated. All the different circumstances of which we have spoken above have not been duly taken into account. Before all calculations, the atmospheric pressure has been deducted; the resistance of the engine, or its increase in proportion to the load, has been omitted; and, above all, the pressure on the piston has been calculated as equal to the pressure in the boiler, though we have seen how different they are from each other. With so many causes of error, it is not surprising that results should have been obtained, which are contradicted by experience; or, in other words, that one should construct very good engines without being able to calculate their power or effects. But if we take into account

all the resistances really conquered, and the velocity of their points of application; if we take the pressure in the cylinder as it really is, instead of considering a power as applied when it is not; in that case we shall obtain a most remarkable result, applicable, moreover, to all sorts of steamengines, viz. that all the power applied is to be traced in the effect produced, and that there is not one single pound of which the use may not be pointed out.

CHAPTER VI.

OF SOME ACCESSORY DISPOSITIONS AND THEIR

EFFECT.

ARTICLE I.

OF THE REGULATOR.

§ 1. Effect of the opening of the Regulator.

Three accessory parts or dispositions are still to be considered, which have a considerable influence on the effect of locomotive engines; these are the regulator, the blast-pipe, and the lead of the slide, which we are going to describe successively.

We have observed that the pipe, which leads from the boiler to the cylinders, may be either completely or partially shut by means of a cock or regulator. When the regulator is quite open, the steam enters into the cylinder as freely as the area of the pipe through which it must necessarily pass. Then the speed is as great as the generation of steam permits. If, by means of the regulator, we diminish

a little the entrance of the pipe, the steam may take at first a greater velocity, which surplus of velocity may allow, as before, the egress of all the steam generated. In that case the effect will remain the same as in the former one, and as long as the width of the passage is not out of proportion with the generation of the steam, there will be no diminution in the effect of the engine.

If, however, we continue to shut the passage, we shall necessarily arrive at last at a point where it will be so narrow, that it will form a considerable obstacle to the admission of the steam. From that moment, only a portion of the steam generated in the boiler will be able to get into the cylinders, and consequently the effect produced will be diminished in the same proportion.

Having called effective evaporating power the mass of steam the engine is able to introduce into the cylinders in an unit of time, we clearly see that the motion imparted to the regulator causes a diminution in the effective evaporating power of the engine; and then the formula, such as we have given it above, shows why the effect is diminished.

In fact, we find in practice that the same train will be drawn by the same engine at different speeds, according to the size of the aperture of the regulator. This is the method invariably used on the Liverpool Railway to prevent the trains, when they are too light, from being carried along with greater rapidity than the preservation of the en

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gines, the carriages, and the railway can allow. This manner of regulating the speed is so far advantageous, that, if on the road there occur either a slight inclination or any obstacle whatever, one may, by opening the regulator, and animating at the same time the fire, restore to the engine its full power, and enable it to pass over the obstacle without diminishing its speed.

The size of the aperture of the regulator is, therefore, to be taken into account, when the question is to ascertain the effect of an engine. That is the reason why we have noted it in the experiments related above. We should have preferred the handle of the regulator to have turned on a graduated circle, in order to be able to measure exactly the degree of opening, and compare it with the corresponding effects; but, with the present construction of the engines, it is only by approximation that we can judge of the size of the aperture.

§ 2. Of the Steam Pipes.

Carrying still further the same principle, on the free motion of the steam, we see that between two engines, perfectly similar in other respects, there must be an advantage in favour of that one in which the steam-pipes have a more considerable area. It is, however, clear, that as soon as we have attained a diameter sufficient for the passage of all the steam that a boiler is able to generate, at the greatest