fire, by means of the bellows, and adopted that of exhaustion, by means of a fan-wheel, applied, in a chamber, at the chimney end of the generator. We have, however, no opportunity of giving correct data of the performance of these engines; and we, therefore, abstain from any opinion whatever as to their merits. An experiment made at Mr. Laird's works, at Liverpool, upon a low-pressure boiler, by an exhausting apparatus, of Messrs. Braithwaite and Erickson's principle, having shewn a surprising result, as to economy of fuel, may induce some to adopt this principle. The length of flues, in this Experiment, (see Note E, Appendix,) was, however, forty-five feet; a length, we should imagine, rather difficult to obtain in a locomotive engine, and to which, we suspect, the economy of fuel in this experiment was attributable.

Both the engines of Messrs. Stephenson and Erickson may, therefore, be said to be on the same principle, viz., that of exhaustion, by mechanical means; the former, by the application of the steam into the chimney, after its passage through the cylinders, and the other by a fan. It remains yet to be ascertained, which, in the first place, produces the most complete exhaustion, and then, which of the two requires the greatest power to effect it; the power required to work the fan, on the one hand, or the loss of power, occasioned by the contraction of the exit pipe, to produce a jet of steam into the chimney, on the other. The principle of exhausting, or producing a current of air for combustion, by mechanical means, allows the whole of the useful heat to be abstracted; none being required to produce a draught in the chimney, as in engines, the process of the combustion of the fuel of which is kept up by the rarefaction of the air in the chimney; and, therefore,

we should expect a considerable effect produced, in the economy of fuel of those engines, the draught of the fire of which is produced by mechanical

means.

These were, generally, the observations made in the second edition of the work, published in 1831. At that time, the improvements, resulting from the Liverpool experiments, were just being carried into practice, and every succeeding engine made was more powerful, and in a higher state of perfection, than that which preceded it. Since that time these improvements have been extended, so as to produce engines much more efficient, more compact, and with evaporating powers more than five times greater, than engines of the same weight were capable of effecting formerly; and their power of dragging loads, compared with their weight, has been comparatively increased.

a.-Explanation of the Principles which govern the Power of Locomotive Engines.

We have already explained, that the power of a locomotive engine, is not to be estimated by the pressure of the steam in the boiler, and the diameter, and length of stroke, of the piston. In passing between the boiler and the cylinder, the elastic force of the steam is diminished, before it reaches the cylinder, by the smallness of the apertures of the steam pipes, through which it has to pass. This difference is, likewise, more frequently produced, by the evaporating power of the engine, not being capable of keeping up a supply of steam to the cylinders, of the elasticity equal to that in the boiler; and, therefore, the pressure upon the piston is less than that against the steam valve of the boiler;

S

and this diminution of the elasticity of the steam, in the cylinders, as compared with that in the boiler, will, in many cases, be in the ratio of the increase of velocity of the engine. Thus, suppose an engine, capable of evaporating a certain quantity of water, per hour, or converting it into a certain bulk, or quantity of steam, of the elasticity indicated by the valve on the boiler ; if this production of steam is sufficient to supply as many cylinders full of steam, of the density of that in the boiler, as shall be equal to the number of strokes, per minute, of the piston, required to produce the given velocity; then, the elasticity of the steam, in the cylinder, will be the same as that in the boiler, except that which is required to force the steam, through the steam passages, with the requisite velocity; and, consequently, the pressure on the piston will be nearly the same as that in the boiler. But, if the velocity of the engine is such, that the number of cylinders full of steam required is greater than the evaporation of the boiler can supply, at the elasticity marked by the steam valve, then the elasticity in the cylinders is correspondingly diminished. Thus, suppose an engine, capable of evaporating 50 cubic feet of water into steam per hour, and that the pressure, on the steam valve, is 50 lbs. per square inch; this will supply a given number of cylinders full of steam of that elasticity. Suppose the resistance, to the motion of the piston, be equal to this pressure of the steam, or equal to the elasticity of 50 lbs. per square inch, of the surface of the piston; then the engine will travel at that rate, which the evaporating power of the engine will supply it with the requisite number of cylinders full of steam. But, suppose the resistance upon the piston increased, by a change in the gradients of the railway, then the velocity

of the engine will be diminished, until the evaporating power raises the elasticity of the steam in the boiler, so as to counterbalance the increased resistance of the piston, and the engine will, consequently, move more slowly. On the contrary, if the resistance be diminished, by a change of the gradients of the railway, then steam of a less density will be required, and, consequently, a greater number of cylinders full will be furnished by the boiler, and the velocity of the engine will be increased.

We see, therefore, that the only correct expression of power of these engines, is the evaporating power of the boiler, and that the velocity, with which the engine will move, will depend entirely upon the quantity of water it can convert into steam, in a given time; or the number of cylinders full of steam, of a given elasticity, which the boiler can produce in a given time. Having found, therefore, by experiment, the quantity of water which an engine, of given dimensions, can evaporate per hour; we then find the power, which that engine is capable of exerting upon the piston, and the velocity, or number of strokes, per minute, which that evaporation will produce, with a given load. The volume of steam, which a cubic foot of water will produce, depends upon the elasticity; this has been ascertained by various experimentalists, and the following Table will shew the result.