The inside and the outside shells of the fire-box are united to each other by a wrought-iron bar or _ring_ (_A F_, figs. 41 and 42) which completely surrounds the inner shell and closes the water-space between the two shells. This bar is bent and welded to the proper form to extend around the bottom of the inside fire-box, and it is riveted to both shells. The water in the water-space is in free communication with the rest of the water in the boiler; and thus the flat sides of the respective shells of the fire-box are exposed to the full pressure of the steam, which tends to burst the outside shell and collapse the inside one. These flat sides, by themselves, would be unable to resist the strain upon them, but as the strain upon the respective fire-boxes is in opposite directions, and necessarily equal for equal areas of surface, tie-bolts, _n_, _n_, _n_, _n_, (figs. 41 and 42,) or, as they are called _stay-bolts_, which are from ³⁄₄ to 1 inch in diameter, are screwed through the plates at frequent intervals, usually from 3¹⁄₂ to 4¹⁄₂ in. apart, so as to connect the two fire-boxes securely together, the ends of the stay-bolts being also riveted or spread out by hammering so as still further to increase their holding power. These bolts, owing to the expansion and contraction of the boiler and other strains to which they are subjected, very frequently break, and if they are made of solid bars of metal there is no way of discovering with certainty whether they are in good condition or not without taking the boiler to pieces. They should therefore be made of the best quality of wrought iron, brass or copper and should also be made tubular, that is they should have a hole through the centre, so that when they break the water will escape at the fracture into the hole and the leak will thus indicate the defect and danger. The latter is much greater from this cause than is usually supposed, and it is not unusual to find on taking a boiler to pieces that a large number of the stay-bolts are broken. [Illustration: _Fig. 41._ Scale ¹⁄₄ in. = 1 foot.] [Illustration: _Fig. 42._ Scale ¹⁄₄ in. = 1 foot.] QUESTION 91. _How can the strain on the flat surface of a boiler between the stay-bolts be calculated?_ _Answer_. By MULTIPLYING THE AREA IN INCHES BETWEEN ADJACENT STAY-BOLTS BY THE PRESSURE. The reason for this is, that each stay-bolt must sustain the pressure on a part of the plate to which it is attached. Thus in fig. 43 it is plain that the bolt _s_ must sustain the pressure on one-half of that part of the plate between it and the bolts _v_, _t_, _w_, _u_, around it, or the pressure on the square _a b d c_, whose sides are equal to the distance (4 inches) between the centres of the bolts. With a pressure of 100 pounds per square inch, the calculation would therefore be: 4 × 4 × 100 = 1,600 lbs. on each bolt. [Illustration: Fig. 43. Scale 1¹⁄₂ in. = 1 foot.]