portionate to each other, and the extra weight, when maintained in motion, becomes an expensive and unwieldly encumbrance. A knowledge of the strength of the materials used, and of sound judgment in its distribution, are therefore among the most essential qualifications of the practical engineer. Our limited knowledge and defective principles of construction are manifest from the numerous abortions which exist, and although I am free to communicate all that I know on the subject, I nevertheless find myself deficient in many of the requirements necessary for the attainment of sound principles of construction.

Reverting to the question more immediately under consideration, it is, however, essential to give the requisite security to those parts which, if left unsupported, would involve the public as well as ourselves in the greatest jeopardy.

The greater portion of the fire-boxes of locomotive boilers, as before noticed, have the rectangular form, and in order to economise heat and give space for the furnace, it becomes necessary to have an interior and exterior shell. That which contains the furnace is generally made of copper, firmly united by rivets, and the exterior shell, which covers the fire-box, is made of iron and united by rivets, in the same way as the copper fire-box. Now these plates would of themselves be totally inadequate, unless supported by riveted stays, to sustain the pressure. In fact, with one-tenth the strain, the copper fire-box would be forced inwards upon the furnace, and the external shell bulged outwards, and with every change of force these two flat surfaces would move backwards and forwards, like the sides of an inflated bladder at the point of rupture. To prevent this, and give the large flat surfaces a degree of strength equal to the other parts of the boiler, wrought iron or copper stays, to 1 inch thick, are introduced; they are first screwed into the iron and copper

on both sides to prevent leakage, and then firmly riveted to the interior and exterior plates. These stays are from four to six inches asunder, forming a series of squares, and each of them will resist a strain of about fifteen tons before it breaks.

Let us now suppose the greatest pressure contained in the boiler to be 200 lbs. on the square inch, and we have 6 × 6 × 200=7,200 lbs. or 34 tons, the force applied to a square of 36 inches. Now as these squares are supported by four stays, each capable of sustaining fifteen tons, we have 4 x 15-60 tons as the resisting powers of the stays; but the pressure is not divided amongst all the four, but each stay has to sustain that pressure; consequently the ratio of strength to the pressure will be as 43 to 1 nearly, which is a very fair proportion for the resisting power of that part. *

We have treated of the sides, but the top of the fire-box and the ends have also to be protected, and there being no plate but the circular top of the boiler from which to attach stays, it has been found more convenient and equally advantageous to secure those parts by a series of strong wrought iron bars, from which the roof of the firebox is suspended, and which effectually prevent it from being forced down upon the fire. It will not be necessary to go into the calculations of those parts; they are, when riveted to the dome or roof, of sufficient strength to resist a pressure of 300 to 400 lbs. on the square inch. This is, however, generally speaking, the weakest part of the boiler, with the exception, probably, of the flat end above the tubes in the smoke-box, if not carefully stayed.

In the flat ends of cylindrical boilers, and those of the marine principle, the same rule applies as regards construction, and a due proportion of the parts, as in those

* For a further illustration of this subject see experiments on Locomotive Boilers, Appendix, No. II.

of the locomotive boilers, and must be closely adhered to. Every description of boiler used in manufactories, or on board of steamers, should, in my opinion, be constructed to a bursting pressure of 400 to 500 lbs. on the square inch; and locomotive engine boilers, which are subjected to a much severer duty, to a bursting pressure of 700 to 800 lbs.

It now only remains for me to state that internal flues, such as contain the furnace in the interior of the boiler, should be kept as near as possible to the cylindrical form: and as wrought iron will yield to a force tending to crush it of about one-half of what would tear it asunder, the flues should in no case exceed one-half the diameter of the boiler. But the force of compression is so different from that of tension, that I should advise that the diameter of the internal flues be in the ratio of 1 to 2, instead of 1 to 2, of the diameter of the boiler; but with our present knowledge on this subject we do not require to restrict ourselves within the same narrow limits that were prudent when the laws of collapse were unknown.

I will not trouble you with a description of the haycock, hemispherical, and waggon-shaped boilers: they are all bad as respects their powers of resistance, and ought to be entirely done away with; I shall congratulate the public when they disappear from the list of those constructions which have the confidence of the man of science as well as that of the practical engineer.

In conclusion, I have to recommend attention to a few simple rules, which, if carefully observed, will lead to the most satisfactory results. To construct boilers as nearly as possible of maximum strength, I have already observed that they should be of the cylindrical form; and where flat ends are used, they should be composed of plates one-half thicker than those which form the circumference. The flues, if two in number, to be of the thick

ness shown in the table; and the flat ends to be of thick plates carefully stayed with gussets of triangular plates and angle-iron, firmly connecting them with the circumference, as in the annexed sketch (Fig. 4). I earnestly recommend the use of gussets as being infinitely better, and more certain, in their action and retaining powers, than stay rods. Gussets, when used, should be placed in lines diverging from the centre of the boiler, and made as long as the position of the flues and other circumstances in the construction will admit. They are of great value in retaining the ends in shape, and may safely be relied upon as imparting an equality of strength to every part of the structure.

I would in conclusion again direct attention to the facts which I have endeavoured to explain. You will, I am persuaded, find them useful; and I trust the objects contemplated by the Committee of your valuable Institution will be fully realised, in the acquisition of greater security, and, I trust, in the attainment of a more perfect knowledge of the principles of construction.

54

LECTURE III.

ON BOILER EXPLOSIONS.

IN a former Lecture I endeavoured to explain the principles on which boilers should be constructed, and the laws which govern the strength and other properties of these important vessels. The subject of construction is one of vast importance, and those forms which give the greatest security with the least quantity of material must embody the true elements of construction, and may be considered as the safest examples for imitation. Boilers, of all other vessels, in the variety of their conditions, shapes, and dimensions, require the head of the philosopher as well as the hands of the mechanic. They contain, within comparatively narrow bounds, a force which, if properly governed, will propel the largest and most stately vessel against wind and tide; perform the work of a thousand hands, and drive a hundred cars loaded with hundreds of tons, at the speed of the swiftest racehorse, from one extremity of the kingdom to the other. They do all this, and more; they impart heat and comfort to our dwellings,-are essential in all the requirements of our domestic arrangements,—and under judicious management, advance the interests of commerce, and contribute to the enjoyments of civilised existence.

Reverse the picture, and entrust the construction and management to the hands of incapacity and ignorance, or to the reckless folly and hardihood of fancied security, and death and destruction follow as a result. When