as the outer shell. They were fixed in the position shown in the annexed diagram, and were intended to resist an ordinary working pressure of only 40 lbs. upon the square inch. In submitting them to the usual test of double pressure, the flues of the first or longest boiler gave way with 97 lbs. upon the square inch; and those of the shorter boiler required 127 lbs. to effect the same distortion. With these large tubes a complete collapse was not accomplished, but the circular form, indicated by the dotted line, was distorted, and the flue became elliptical, as shown at b b*.

Fig. 3.

b

The weakness of the flues in the above

experiments is so evident as to need no comment.

To

* Reducing the above results to unity of length, which with flues of this size should give a nearly constant quantity, we have

The correspondence in the last column shows that these flues obey the law of inversely as the lengths, very nearly, in their powers of resistance.

It may be well to test the accuracy of the formula which has been found to apply to tubes of a length not greater than 10 feet, by determining from it the strength of flues similar to the above, and comparing the results with those derived from experiment.

Here, for the boiler 35 feet long, we have by formula

This difference confirms the view already stated, that the formula for short tubes does not apply strictly to tubes longer than 10 feet.

For the boiler 25 feet long, we have

P=109 lbs. ; by experiment 127 lbs.

A less difference between the experimental and calculated result, as would have been anticipated from the shorter length of the flue.

It will be observed, that even these experiments, upon full sized boilers, are remarkably consistent, and offer no discrepancies which cannot be easily explained consistently with the general formula.

remedy it, it has been already stated, we need only resort. to a construction so simple, and yet so effective, as to meet at a small expense all the requirements of the case.

Figure 1, Plate II., exhibits an ordinary boiler flue, 30 feet long and 2 feet 9 inches in diameter, with simple lap-joints, as hitherto invariably constructed. To attain nearly three times the strength of this, it will only be necessary to introduce two or more strong, rigid, angle or Tiron ribs, as exhibited in figs. 2 and 3, at a, a. This arrangement will not only remove all doubts as to the strength of these flues, by bringing them within the limits to which the formula applies with strictness, but will give to flues 30 feet long a strength equivalent to that of flues only 10 Feet long, and make them uniform in their powers of resistance with the other parts of the boiler.

The reduction of the strength of flues by the lap-joints has already been stated; the deviation from the true ylindrical form which they cause, lessens, in some cases, eriously the strength of the vessels, as may be seen in Experiments 23 and 24, Table VI. Hence it is also proposed that flues required to resist an external pressure hould be formed with double-riveted butt-joints, with ongitudinal covering plates, as shown at b, b, b, fig. 3, Plate II. It is believed that these alterations will secure mple safety in these important constructions, and in this rust they are commended to the attention of the engineer nd the public generally.

II.

RESEARCHES ON THE RESISTANCE OF GLASS GLOBES AND CYLINDERS TO COLLAPSE FROM EXTERNAL PRESSURE; AND ON THE TENSILE AND COMPRESSIVE STRENGTH OF VARIOUS KINDS OF GLASS.

(Reprinted from the Transactions of the Royal Society, 1859.)

THE recently published experiments upon the collapse of tubes of wrought iron, led to results so novel and so much at variance with the ordinary rules of practice, as to exemplify anew the caution and diligence which are requisite in investigating the physical laws of nature, in order to arrive at just conclusions in regard to the properties of materials and their most effective distribution for the purposes of construction.

In the experiments alluded to it was clearly shown that the prevailing ideas of the strength of vessels subjected to a uniform external force were erroneous, and at variance with the laws of resistance to collapse under such circumstances; whilst in practice the prevalence of error in this matter had led to serious and sometimes fatal accidents, arising out of the construction of vessels of inadequate strength to sustain the pressures placed upon them. These errors, it is hoped, need no longer be perpetuated, and in order to give them as wide a circulation as possible, I have with the permission of the Council of the Royal

*These experiments were carried on in conjunction with T. Tate, Esq., but are reprinted here on account of their connection with, and confirmation of, the experiments in the preceding paper.