fire-place to be three or four feet high, here we have first of all a tolerably large item of bulk to be conveyed along with the vehicle.

But besides the inconvenient size of the boiler, its weight is necessarily considerable; for, in order to prevent explosions, its sides must be thick and strong iron plates, and by so much the stronger and the heavier, as the engine is made larger and more powerful. First of all then, the boiler must have size to give power, and weight to give strength. These two circumstances form the horns of a dilemma between which the invention long stuck fast. The boiler was first made large to gain the necessary power, but in making it strong it became so heavy that the engine had scarcely power sufficient to drag its own weight, and became of no practical use. On the next experiment, having found the boiler too heavy, it was made smaller to diminish the weight, and thus its dimensions were inadequate to work the engine; or lastly, being made both large enough and light enough, but on that account too thin and weak, an explosion was the consequence, and the machine was blown to pieces. How then is a boiler to be made large enough and light enough without being dangerously weak? This is the difficulty of the first part of the problem.

The

Several methods have been adopted to render boilers at once powerful, light, and safe. Indeed the improvement of them in these respects has been the subject, perhaps, of more inventions and patents than any other part of the steam-engine. majority of these inventions are arrangements upon this principle, that the power of a boiler does not depend so much upon its own size, as upon the size of the fire and the extent of the part of the boiler on which it acts: that if the fire be put not only below the boiler, but around it, so as to heat it on every side, it will generate a quantity of steam by the sides as great as that which is produced from the bottom; that, in short, the effect of a boiler is proportioned, not to the quantity of water it contains, but to the surface it exposes to the action of the flame, and the manner in which the waste of heat is prevented. This is effected in the simplest possible way, by passing the flame round the sides of the boiler before allowing the hot air and smoke to ascend the chimney. A more compact form of boiler has been made by placing the fire and chimney within the boiler itself, so that the boiler should surround the fire, instead of being surrounded by it, and thus the escape of useful heat has been very much avoided; this is the method used in constructing the boilers of steam-ships, in which the fireplaces and ash-pits may be seen in the middle of the boiler,

and the smoke passage is made to wind round the inside of it, before it joins the upright chimney. In this method and the former one the boiler is very large in its outer surface, and being on that account weak, requires to be thick, and consequently heavy. A further improvement upon these boilers forms what have been called, from their construction, tubular boilers. These consist of numerous small pipes containing water, some of them passing through the middle of the fire in its hottest part, others forming the bars on which the fire rests, and a third class receiving the heat of the flame immediately above the fire; all of these send their supply of steam to a reservoir above, from whence it passes to the engine. An equal effect may be obtained by penetrating a boiler of considerable dimensions by a number of small tubular passages or flues, along which the flame may be conducted to the chimney, and give its heat to the water in its course. By multiplying the number of small surfaces exposed to the fire in tubes or thin small chambers, mutually connected by various ways, boilers are in fact made strong, powerful, and light; and it is by one or other of these methods that the boilers of steam-carriages have been rendered more nearly perfect than any other part of their mechanism.

2. On the supposition that all difficulty in generating a supply of steam has been conquered, the next object will be to make the best use of it in moving the carriage, so that the quantity wasted may be the smallest possible. It is only when the steam passes from the boiler into the cylinder that it comes to act upon the solid machinery, and put its parts in motion. The cylinder confines the action of the steam to the surface of the solid piston, which it presses first downwards and then upwards, thus turning round the wheels. Now the steam is conducted from the boiler into the cylinder at the top and bottom by means of a pipe, which is often made of considerable length, for the convenience of having the weight of the boiler in one place and that of the cylinder at another, and in this passage the pipe frequently makes more than one turn. A fresh difficulty here arises; the nature of steam is such, that a passage, if it be either long or narrow, or have any turns, greatly diminishes the force of the steam, and a very serious loss may be thus incurred; a single turn in the direction of a pipe will deprive the steam of one-tenth of its power, and every successive turn of a similar portion. If, therefore, the direct pressure of the steam in the boiler be such as would raise 1000lbs., and it had to turn one corner before entering the cylinder, it would only raise 900lbs.; while four or five turns would reduce that amount to one-half,

No engines have yet been made that have less than two or three such turns to encounter, and hence one-fifth or one quarter of their power is always wasted; so that in an engine of 500 horse power, the power of more than one hundred horses is nearly thrown away. In the construction of steam-carriage engines this principle has been lost sight of so monstrously, as in one case to have deprived the engine of its proper power to the extent of four parts out of five of the whole.

3. On the supposition that an inventor has succeeded in mastering these difficulties, greater and more serious ones still present themselves. The form and size of the cylinder will materially affect the speed of the engine. The present rage appears to be for long and narrow cylinders. We have seen a carriage as large as one of Wombwell's caravans, and not less weighty, furnished with two little cylinders, each 4 inches in diameter and 15 inches long; and this carriage consisted of two floors, one above another. In general the cylinders are made too long and too narrow, thus exposing much unnecessary surface to the effects of friction and cooling. It is also usual to give the engine two cylinders instead of one. We are of opinion that one cylinder is equal in capacity and preferable to two. We know, both from theory and experience, that it is more powerful; and as we know that it is impossible for two horses to pull as much together as either would separately, so is it also impossible that two cylinders can act together with perfect precision. If one cylinder do not in each stroke give power enough to pass the line of centres, there is very little use in trying the experiment at all.

4. There is yet another requisite which is quite indispensable to the success of the steam carriage-namely, an arrangement for supporting the carriage-body and the whole of the moving machinery upon perfectly flexible springs, so as to vibrate freely in every direction, and yet admit of being impelled forwards with uniform power and velocity. To apply a continuous force to a pair of wheels through a set of springs, from a machine that is permitted to swing backwards and forwards, as to be now nearer to them and then farther off, implies a combination of stiffness with flexibility that seems an absolute contradiction; it requires that those parts should be rendered moveable which it is of the greatest importance in a stationary engine to preserve immoveable. From the necessity of this provision, it has been attempted, and professed to have been accomplished in almost every instance of the invention; but in every carriage hitherto brought on the road (as we shall afterwards show) the attempt has failed. We say failed, uot in the circumstance that there are no springs, but in the fact that

either the whole of the weight does not rest upon them, or their action is not permitted. In some, the body of the vehicle is set on springs, but not the machinery; in others, the whole is first set on springs, and then, upon finding their action inconsistent with the other mechanism, these are trammelled and tied down to prevent their bending, or so strengthened, thickened and shortened, as to be little more than rigid blocks of metal. We shall not at present give the solution of this difficulty, but we think it may not be impossible to put our readers in the way of comprehending its precise nature. They already understand how it is that the engine propels the carriage in turning round its wheels by a rod or arm proceeding from the engine, and acting on a handle of these wheels, as the arm of a man would act on the wheel of a crane to turn it round. Now if the engine rested on springs placed between it and the wheels, it would sometimes approach, and at other times recede to a variable distance from them; otherwise, each jolt, by drawing the engine on one side or other of the wheel, or backwards and forwards, has the effect of pulling the wheel first in one direction and then in another, rendering the motion unequal, and the progress of the vehicle irregular and desultory. The only plan introduced to prevent this is imperfect suspension, and imperfect suspension has been the ruin of every machine that has yet been constructed. When a heavy mass like a carriage of from four to eight tons weight is put in motion on a rough road, every stone which it meets communicates a shock to the whole of the machinery and the vehicle; this shock is productive of two evils: first, it deprives the mass of a part of its motion, so as both to diminish its velocity, and render a greater force necessary to continue the motion, and then it is obvious that a series of shocks constantly repeated upon machinery so delicately adjusted, and yet so heavily strained, as the steam-engine, must greatly injure the parts, and rapidly destroy them. Continued jolting, indeed, is the most certain mode of separating and deranging the parts of solid mechanism, as it loosens the bolts and screws that keep them together; it is in fact the operation of the same principle resorted to when we wish to detach the stopper of a wine decanter that may have stuck fast in its place, we give it a few smart taps with the handle of a knife on alternate sides, and find this more effectual in loosening its hold than a great force directly applied to draw it out. The effect of springs ought to be, to detach the vehicle and its load from the wheels and axles so effectually that the jolts received by them shall not be communicated to the superincumbent weight; and unless this be fully and extensively done in every case, future experiments will share

VOL. X. NO. XX.

LL

the fate of the past. The carriage may make one trip successfully, and perhaps a second one if short, but disabled by the series of jolts it has sustained, it will proceed no further till thoroughly repaired, and so on for every journey till it is shaken to pieces.

5. Another difficulty, the last we shall mention, is to construct an engine of variable power like that of a horse, which shall proportion its exertion to the resistance to be overcome, using on a level road no more force than will move the given load with the requisite velocity; increasing this force in the degree necessary to ascend an elevation; using a greater for a greater degree of steepness, and a less for one more moderate, and again reserving the force going down hill, so as to use it with effect on the next emergency. This has never yet been done. Do we require to say, that such an accommodation is indispensable to success?

These five parts of the problem must, therefore, be separately accomplished in the highest degree of perfection, and then combined in one compact and uniform whole, before we can expect perfect success in any attempt at the construction of steam-carriages. If any one of these circumstances be neglected or imperfectly accomplished, even although all the others should be completely obtained, that one omission will be fatal. We must have a boiler at once strong and light, containing space for a large fire, an extensive heating surface, and a capacious reservoir. The supply of steam must be economised to the greatest possible extent, by widening the passage-pipe, shortening, and making it straight; the cylinders to which it is conveyed must be so proportioned as to give the greatest possible benefit from their form, position, or number, and the simplicity of their appendages. And while the utmost rigidity must be sustained among these moving parts, to ensure their operation, they must yet be allowed such a measure of vibration in every direction, that being hung on perfectly flexible and highly elastic springs, they shall be allowed to act upon them either upwards or downwards, backwards or forwards, to the right hand or to the left, without in the slightest degree affecting the uniform velocity of the carriage; finally, a provision must be made, by altering the force of the steam or its quantity, or otherwise arranging the parts of the carriage, for giving on different kinds of road such degrees of power as may impel the vehicle at a velocity nearly uniform, whether ascending or descending, or running on the level. If this construction be possible, and we have little doubt but that it is so, then we may still expect to see the invention fully perfected. Certainly, if we find, in pursuing our investigations, that every carriage hitherto produced has been deficient in one or other of these essential points, we think that it will go