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LECTURE VIII.

ON THE PROPERTIES OF STEAM, ITS MANAGEMENT AND APPLICATION.

[Delivered before the Leeds Philosophical Society. March 1860.] Ir we were to enter upon the statistics of the accidents which have arisen from the use, or rather the abuse of steam, we should have to present a fearful catalogue of catastrophes and loss of life. Few people are fully aware of the suffering which these accidents have entailed upon a certain class in the community, or of the immense destruction of property which has proceeded from the misdirected employment of steam.

The Great Author of Nature has supplied us with a power which properly applied changes the destinies of nations, and confers unheard of benefits upon the human race. We have it at command in every department of human effort, and in every condition of life it subserves our will through every gradation from one to a thousand horses power. It stems the tides and currents of the ocean. It ploughs, spins, weaves, and grinds our corn. It drains mines, pumps water, and carries us across the country with a celerity unknown in the past, and with a despatch and power which would have set at defiance the Genie of the Arabian Nights. It only requires a careful and judicious treatment, in preserving it from excesses of heat and cold, and confining it within bounds sufficiently strong to retain it, and a wise direction of its efforts, founded upon a clear and accurate knowledge of its properties.

When we look around us and count the number of steam engines at work in every city, town, and hamlet; when we see them traverse the rail, and depart across the ocean with unerring certainty, and consider at the same time the necessity of having all these movements under safe control, it assuredly follows that the makers of these engines and those who employ them should bring to their construction and superintendence a large degree of intelligence and a full knowledge of their principles. Unfortunately this is not always the case; we are still deficient of knowledge on many points of construction and in regard to many of the properties of steam. To supply part of that knowledge is the object of the present address, in which I hope to show some of the means by which steam boilers may be made more secure, and to correct some erroneous views which have obtained currency in regard to the properties of steam.

I. NEW PRINCIPLES IN THE CONSTRUCTION OF BOILERS.

The production of steam in a vessel placed over a fire is a simple and well-known process, and one which requires no comment. The form of the vessel, commonly called the boiler, when steam is to be produced for industrial purposes, is, however, an important consideration, both as regards economy and strength. In the history of the steam engine we find an immense number of forms of boilers, most of them having served the purposes for which they were intended. At first, when high pressure steam was scarcely known, great strength was not required, and the form of the boiler was not of so great importance as at present. At that time the supply of water was regulated by a float, and the pressure in the boiler seldom exceeded 10 or 12 lbs. per square inch. At this pressure the waggon-shaped boiler and other forms with flat surfaces were not so objectionable; but when steam is employed at from five to fifteen times this pres

sure, it becomes a question of the highest importance that the vessels to retain such an immense force should be constructed of the best material, duly proportioned in thickness, and arranged in the form of maximum strength.

In former lectures I have discussed the rules and proportions for the construction of boilers adapted for stationary engines, especially in reference to the resisting powers of the outer shell. At the time that those proportions were given we were not aware of a hidden source of weakness, which further investigation has discovered, and which we now propose to remedy by simple means, which have been suggested by an increased knowledge of the laws of construction.

It is well known that the great majority of the boilers in this country are now constructed with internal flues or

Fig. 69.

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tubes traversing the whole length of the boiler, most of them being upwards of thirty feet long, and varying from two to three feet in diameter. These flues are generally composed of plates of the same thickness as the outer shell of the boiler, and being of cylindrical form, they have been considered hitherto much stronger in their powers of resistance when forming an arch opposed to a uniform external pressure, than the outer shell subjected to the same force acting from the interior. This opinion was acted upon with confident security until its erroneousness was shown by direct experiment. Fig. 69 shows the ordi

nary form of the boiler in the manufacturing districts,

about 30 feet in length, 7 feet in diameter, with two in

ternal flues a a, about 2 feet 8 inches in diameter. These flues as well as the external shell are generally composed of 용 inch plates, which corresponds with a bursting pressure for the outer shell of 3034 lbs. per square inch, which of course ought at the same time to be the collapsing pressure of the flues. This unfortunately is not the case, as, according to experiment, it is now found that the flues would give way with about 100 lbs. pressure per square inch, or the boiler would be destroyed by collapse at onethird of the bursting pressure of the shell. It is evident then that these internal flues have been constructed a great deal too weak, and without a knowledge of the true law of collapse.

It has long been a desideratum to obtain some law by which the engineer could proportion the strength of the internal flues. There have been no definite rules to guide us hitherto in proportioning the diameter, length, and thickness of plates of the flues, so as to correspond with the strength required in the boiler. And even in cases where explosions have taken place from collapse, we have, it is to be feared, too often mistaken the actual cause, from the quantity of the débris covering the site, and the force which has torn to pieces the outer shell.

To supply this want I undertook some time ago a long series of experiments on the laws of collapse, the results of which were made public through the Transactions of the Royal Society.* The chief laws which were ascertained may be stated as follows:

I. Strength as affected by length. The results under this head are singularly interesting and conclusive.

*The two papers on this subject are reprinted in the present volume, pp. 1—45 and 74-92. In this Lecture the results are exhibited in a more popular form for practical readers.

Within the limits of from 1.5 foot to about 10 feet in length, it is found that the strength of tubes similar in other respects and supported at the ends by rigid rings varies inversely as the length.

Thus taking the 4-inch tubes of different lengths, we have the following mean results derived from experi

ment:

1. Resistance of four-inch tubes to collapse.

The remarkable differences which exist in the resisting powers of the above tubes will be at once apparent. The constancy in the numbers in the last column, which represents the resisting powers of the tubes reduced to unity of length, on the assumption that the strength varies inversely as the length between the supported ends, is a proof of the substantial accuracy of the above law.

2. Resistance of six-inch tubes to collapse.

In this case, as before, the product of the collapsing pressure by the length is constant, and verifies the law.

3. Resistance of eight-inch tubes to collapse.

4. Resistance of ten-inch tubes to collapse.