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account the surplus of power necessary to move the engine itself on a road full of asperities.

Besides, the repairs of the engines are, even on railways, a considerable expense. At Liverpool, of the 30 engines belonging to the company, ten only are in activity on the line for the conveyance of goods and passengers. The effective work is eight or ten hours a-day, and the expense for maintaining in activity those ten engines, amounts to more than £18,000, or £1,800 a-year for each of them. These expenses are paid and become a source of profit, because on a railway the engines draw considerable trains; but it would not be the same thing if the trains were reduced, or, in other words, if a greater number of engines were required to do the same work. Moreover, if the engines, instead of sliding without jolts on the smooth surface of a railway, were obliged to run on the rough soil of our roads, how great would not be the expense of repairs. And we have 12 times as many engines to repair.

Outlay and interest of capital for engines, salary of enginemen and assistants, fuel, repairs, all these articles will soon have absorbed the expected

economy.

Besides, the chief advantage of such undertakings, consists in the speed with which the haulage is executed. When the 29 miles between Liverpool and Manchester were travelled in four hours, there were about 450 passengers going daily

from one of those towns to the other. At present when, thanks to locomotive engines, the journey is completed in an hour or an hour and a half, there are 1,200 passengers a-day. The speed has the greatest share in the creation of that profit. It must be given up if the engines are only to run eight or ten miles an hour.

Now, the 8 or 9 t. that the locomotive engines weigh on railways, allow us to give them a sufficient extent of boiler to generate a certain quantity of steam per minute, and consequently a certain speed. If the nature of the road obliges us to reduce the weight of the engine to 3 t. only, with the necessity of making all its different parts stronger, on account of the jolts on a rough surface, there will naturally be less heating surface in the boiler, and consequently less possible speed. And, in fact, the steam-coaches scarcely do more than eight or ten miles an hour.

As a last reflection, we shall add, that until the present moment the success of locomotive engines on common roads, continues, as a speculation, to be very uncertain, whilst the prosperity of railways, whatever be the moving power, is demonstrated by their continued extension. Steam-. coaches may be improved, but, we repeat, whatever be the advantages they may offer on a common road, it is not to be contested that, by employing them on a railway, those advantages will be infinitely greater.

CHAPTER IX.

OF THE FUEL.

§ 1. Of the Consumption of Fuel in Proportion with the Load.

We have still an important article to discuss. That is the fuel.

From what we have said above, the steam, generated in the boiler at whatever pressure it may be, takes, in passing into the cylinder, a pressure exactly determined by the resistance on the piston. The mode of action of the engine, is thus limited to the transformation of a certain quantity of steam, drawn from the boiler, and consequently at the pressure of the boiler, into steam at a lower pressure and of a proportionally greater volume.

Let us suppose the same engine, with the same pressure in the boiler, and travelling the same distance with two different loads. The distance travelled being the same, the number of turns of the

wheel, and consequently of strokes of the piston or cylinders of steam expended during the journey, will be the same in the two cases. If the load had been the same, there would also have been identity in the nature of the steam expended. But as the loads differed, the same number of cylinders will indeed have been expended, but the degree of the steam in the cylinders will be different in the two

cases.

Then the expense of moving power will be in one case a certain volume of steam at the pressure R, for instance, and in the other case the same volume at the pressure R'.

The pressure of the steam in the boiler being supposed the same in the two experiments, its temperature will also be the same. As the temperature experiences no reduction during its passage to the cylinders, the pipes and the cylinders themselves being immersed in the boiler, or surrounded by the flame of the fire-place, the temperature of the steam in the cylinders will be the same in the two cases.

Thus the volume and temperature of the steam expended during the journey will be the same in both circumstances. The pressure of the steam in the cylinder will alone have undergone a change. Consequently the mass or weight of steam expended, will be in each case in the ratio of the pressure in the cylinder.

The weight of the steam being equal to that of

the water that generated it, the weights of water evaporated will then be to each other as the pressures in the cylinder, or, in other words, as the resistances on the piston. Besides, as the water is first transformed into steam at the pressure of the boiler, that is to say, in both cases into steam at the same degree of pressure, it follows also that the quantities of fuel necessary for the evaporation, will be to each other as the pressures or total resistances on the piston.

This shows that the consumption of fuel is independent of the speed, and that it depends only on the resistance on the piston.

If in the two journeys we consider, the pressure happens not to be identically the same in the boiler, there will be a little more fuel consumed in that case where the pressure has been the greatest, because the pressure could only increase in consequence of an increase of temperature. But as degrees of pressure very distant from each other are produced by very similar temperatures, the difference of consumption occasioned by that circumstance will be of little importance, and will not be perceived in practice.

This principle gives the proportions of the consumption of fuel for the same engine with different loads, and may thus serve to determine its consumption in all circumstances, as soon known in one determined case.

as it is

If for instance Q and Q' are the quantities of

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