Whereas, in the joint marked b, the expansion or contraction of each length of the tube is provided for by the arched or the corrugated piece, and here there is increased strength combined with power of expanding or contracting freely. In the joint c, which is known as "Adamson's flanged joint," there is the strength of the T-iron directly combined with the curved end, which allows of unimpeded expansion or contraction.

The arrangement is most convenient and effective, and is particularly valuable as giving a seam where the rivets are protected from the furnace gases, and are, in fact, immersed in water, one consequence of the construction being that the joint will bear intense heat much better than any other where the rivets are exposed.

STRENGTH OF BOILERS.

While the pressure acts from the centre radially out to the circumference on an indefinite number of radial lines, the mathematics of the strength of the shell supposes that the pressure acts as the arrows a, a, a, a, in Fig. 45, that is, tending to lift the top from the bottom. In Fig. 44 the pressure represented by arrow a is resisted by arrow a', and similarly arrow b and b' resist each other, and as each is equal, we may assume any one direction as that in which the pressure acts, as a, a, a, a, in Fig. 45, tending to

part the boiler through the line b, b. We might, as stated above, assume any other direction of the pressure, but as the shell is as strong through b, b as it is through any other similar section, the strength as ascertained at b, b will be the strength of the boiler. The standard of compar

ison between different brands of boiler iron is based on the strength of a square bar of that iron, the sides of which are one inch in length, and a section through the bar representing one square inch. The tensile strength of such a bar will average 50,000 pounds. Assuming, then, that the pressure tends to tear the top from the bottom of the shell, it is apparent that the force acting to do this is represented by the length of the shell in inches, multiplied by the diameter in inches and by the pressure per square inch. The iron being the same thickness throughout the shell, we will also assume that a section one inch in length of the boiler is a fac-simile as to strength of any other section of similar length. The iron being of an inch thick and one inch (assumed) in length, it is plain that its strength is but % of our standard of comparison, 50,000 (or 15,625 pounds). Now, as we have two sides of the boiler off thick, we must double the above amount, which = 31,250 pounds, as the strength of our section if it were without a joint. The strength of a singleriveted seam (Fig. 46) has been ascertained to be

=

but 56 per cent. of the solid iron, and hence we must take that percentage of 31,250 to arrive at the strength of the weakest portion, which, of course, comes under the principle that the strength of any structure is equal only to its weakest part. 56 per cent. of 31,250 17,500, which is the product of the length of our section (1 inch) × by the diameter, say 48 inches, multiplied by the pressure per square inch necessary to burst the boiler. We have, therefore, in order to find the pressure per square inch necessary to burst the boiler, only to divide 17,500 by the diameter (the length being 1 inch or unity). This gives us nearly 365, which, divided again by our factor of safety, we will take as 6 (or the working pressure to the bursting pressure), and we obtain 60 nearly, as the proper working pressure.

The strength of a joint, like Fig. 46, through the rivet holes, may be found by taking the length a, less the diameter of one hole multiplied by the number of holes. This leaves the length of solid iron between the holes, which, multiplied by the thickness and by the strength of a sectional inch, and from 15 to 20 per cent. deducted from the result, for injury done the iron by the punch, leaves the probable strength of metal between the holes. A staggered or double-riveted joint, shown in Fig. 53, has 70 per cent. of the strength of the solid sheet, as as

certained by Fairbairn's experiments. From inspection, it will be seen that there is as much metal between the holes h, c and h, f as there is between c, f. The metal between the holes is much greater than in Fig. 46, while the number of rivets is in proportion to the metal left between the holes, and hence the strength of the joint may be deducted from the metal left between the holes on a line, a, b. The strength of a rivet, to resist shearing, is about 50,000 lbs. per square inch, hence the sectional area of one rivet, multiplied by the number of rivets, and again multiplied by 50,000, will closely approximate their shearing or detensive strength. The rivet area strength should be kept as nearly equal as possible to the strength of sheet between the holes. The strength of the sheet to resist

crushing is represented by Fig 47. It is plain

that if the rivet does not shear, nor the sheet rupture between the holes, the part a must be pushed or sheared out of the way, if rupture takes place. The resistance which the part of piece a offers is represented by the area b, b, or the length d, by the thickness of the plate plus the equal area c, c, and the whole multiplied by the detensive strength of the sheet, which is about 50,000 lbs. per square inch. In designing a joint, the piece a, Fig. 47, the shearing strength of the rivets and the strength between the holes should be as nearly equal as possible, to make

as strong a joint as is possible. The joint shown in Fig. 51 has a welt piece a, added, and is much used in boiler work. It does not add as

b

FIG. 44.

much strength as is commonly accredited to it. The line of rivets d and f, Fig. 48, is commonly spaced twice as far as the middle one e, and therefore presents only half the shearing

strength of rivets that line e does, and hence if rupture were to take place it would occur first through either d or f, if it were not for the stronger joint e. Therefore, after the internal