which something will be said after the disturbing action of heat has been noticed.

EFFECT OF HEAT.

The wear and ultimate strength of a boiler are greatly influenced by adopting a construction which shall provide for the inevitable changes of form caused by unequal expansion and contraction due to changes in temperature.

Heat is motion, and as soon as the fire inside a furnace flue is lighted, the metal on the top becomes more heated than under the surface, and the tube arches itself in consequence of the greater expansion of the hotter portion.

And not only so, but the flue lengthens as a whole, and the flat ends bulge outwards. Finally the water becomes heated and the whole structure elongates, and unless sufficient allowance be made for the pulsating movements, straining will occur, which may possibly end in rupture.

The linear expansion of wrought iron (soft forged) under the action of heat is stated to be .0012204 for a rise in temperature from o° C. to 100° C.

Thus a bar of iron 30 feet long expands about inch for a rise in temperature of 132.2° C. The expansion of the parts of a boiler as caused by heat is of course capable of accurate measurement; and in particular the so-called

hogging of a boiler flue has been observed by applying three gauge rods attached at equal distances along the crown of the tube; each rod is carried vertically upwards, and passes through a stuffing box in the shell of the boiler, whereby it has been possible to observe very accurately the distortion of the flue.

One boiler experimented on was 28 feet long, and it was found that the flue rose 3% inch when the flame passed around the boiler in the ordinary way along the side flues, and that it rose 1⁄2 inch when the flame was carried directly into the chimney without heating the outer shell.

The gauge-rod at one-fourth the length of the boiler from the front end rose as much as that, and in one case in. more.

Also the colder the water at starting the greater was the distortion, and generally the action was more severe just after the lighting of the fires. As soon as the whole of the water became permanently heated the gauge-rods retired to their primary position, the distortion of the flues seldom lasting for more than an hour. Mr. Fletcher recommends that the end plates of boilers, to be used at a pressure of 75 pounds per square inch, should be 1⁄2 inch in thickness, increasing to inch for increased pressure within moderate limits, excessive thickness being undesirable, as confining or restraining the necessary movement of the furnace tubes. The object

is to strengthen the end plate, and yet to preserve its elasticity; and in carrying out this intention it is a rule to attach the plate at the front of the boiler to the shell by external angle iron. This mode of construction is not, however, adopted at the opposite end.

The furnace flues are a vulnerable part of the boiler, inasmuch as they are liable to yield by collapsing unless sufficiently strengthened.

The subject of strengthening the internal tubes of the internal fire-flue boiler was investigated by Sir W. Fairbairn, whose experiments led to the following conclusions:

(1) The strength of a tube to resist collapse by external pressure is inversely as its diameter. (2) The strength varies inversely as the length.

(3) The collapsing pressure in pounds per square inch

=806300X

2.19

(Thickness of plate in inches.) Length in feet X diameter in inches.

In these experiments the ends of the tubes were firmly attached to rigid plates, and the vessel in which the compressing force was applied was a cast-iron cylinder 8 feet long, 28 inches in diameter, and 2 inches thick, which could be safely strained as far as 500 pounds per square inch.

Into this cylinder air was forced by a pump,

and produced any required pressure on the surface of a quantity of water which nearly filled the cylinder, and in which the tube under trial was completely immersed. There were two gauges for reading the pressure, and a safety valve in addition, which was loaded by a weight.

The experiments made by Sir W. Fairbairn were valuable as calling attention to a material subject connected with the construction of boilers; but it appears that a special contrivance

علي

Fig. 2.

FIG. 42.

FIG. 41.

FIG. 43.

for strengthening furnace flues, while allowing at the same time for their expansion and contraction, had been originated some years prior to the researches to which reference has been made. At a meeting of the Institution of Mechanical Engineers held in 1876, Mr. Adamson stated that he had first employed a "flanged seam," as it is termed, for the strengthening of furnace flues, as early as the year 1851.

The Figs. 41, 42 and 43, exhibit three forms of joints as applicable to tubes subjected to external pressure.

The first, marked a, consists of a ring of T-iron, riveted as shown. It is abundantly strong, and is in a form which has been adopted for centuries past in strengthening guns. The weakness of a tube to resist a bursting pressure on a longitudinal section has been already demonstrated, and a common method of strengthening it has been to apply parallel rings at intervals along its length.

In this way steam cylinders of great length, which are subjected alternately to a bursting and compressing pressure, have been strengthened. Every one is aware of the accession of strength caused by the flange of a pipe. Since action and reaction are equal and opposite, we might have anticipated that a form of tube constructed so as to withstand a bursting pressure from within, would also be the form best adapted for resisting a collapsing pressure from without.

The difficulty of calculation in the latter case arises from the liability to deformation, which is soon set up, after which theory is of little use in enabling us to predict a result.

But although the joint a has ample strength, it is deficient in another quality which is of importance, viz., it does not permit any alteration of length. The whole furnace tube is rigid, and expands or contracts as one piece.