high efficiency, in some cases as high as 94.6 per cent. Fig. 100 illustrates a joint of this kind, and the dimensions are as follows:

main plate and both the inside and outside cover plates or butt straps.

The two outer rows reach through the main plate and inside cover plate only, the first outer row having twice the pitch of the inner rows, and the second outer row has twice the pitch of the first.

Taking a strip or section of plate 15 in. wide (pitch of outer row), there are four ways in which this joint may fail.

1. By tearing of the plate at the outer row of rivets. The resistance is P" - DxTx T.S., or 15-.9375 × .5 × 60,000 = 421,875.

2. By shearing eight rivets in double shear and three in single shear. The strength in resistance is 19A x S,

or .69 × 19 × 45,000 = 589,950.

3. By tearing at inner rows of rivets and shearing three rivets. The resistance is P"-4D x T x T.S. + 3A x S, or 15-(.9375 × 4)×.5 × 60,000+.69 × 3 ×45,000 = 430,650.

4. By tearing at the first outer row of rivets, where the pitch is 71⁄2 in., and shearing one rivet. The resistance is P" - 2D x T x T.S. + A x S, or 15 - (.9375 × 2) x.5 x 60,000+.69 x 45,000 = 424,800.

It appears that the weakest part of the joint is at the outer row of rivets, where the net strength is 421,875. The strength of the solid strip of plate 15 in. wide before drilling is P" x Tx T.S., or 15 x .5 × 60,000 = 450,000, and the efficiency is 421,875 × 100 ÷ 450,000 = 93.7 per cent.

Staying Flat Surfaces. The proper staying or bracing of all flat surfaces in steam boilers is a highly important problem, and while there are various methods of bracing resorted to, still, as Dr. Peabody says, "the staying of a flat surface consists essentially in holding. it against pressure at a series of isolated points which are arranged in regular or symmetrical pattern." The cylindrical shell of a boiler does not need bracing for the very simple reason that the internal pressure tends to keep it cylindrical. On the contrary the internal pressure has a constant tendency to bulge out the flat

surface. Rule 2, Section 6, of the rules of the U. S. Supervising Inspectors provides as follows: "No braces or stays hereafter to be employed in the construction of boilers shall be allowed a greater strain than 6,000 lbs. per sq. in. of section."

The method to be employed in staying

FIGURE 101.

a boiler depends upon the type of boiler and the pressure to be carried. Formerly when comparatively low pressures were used (60 to 75 lbs. per sq. in.) the diagonal crow foot brace was considered amply sufficient for staying the flat heads of boilers of the cylindrical tubular type, both above and below the tubes, but in the present age, when much higher pressures are demanded, through stay rods are largely employed. These are soft steel or iron rods 14 to 2 in. in diameter, extending through from head to head, with a pull at right angles to the plate, thus having a great advantage over the diagonal stay in that the pull on the diagonal

FIGURE 102.

tay per square inch of section is more than 5 per cent in excess of what a through through stay would have to resist under the same conditions of pressure, etc.

The method of calculation for diagonal bracing is given in Chapter I and will not be discussed here.

The weakest portion of the crow foot brace when in position is at the foot end, where it is connected to the head by two rivets. With a correctly designed brace

the pull on these rivets is direct and the tensile strength of the material needs to be considered only, but if the form of the brace is such as to bring the rivet holes

FIGURE 103.

above or below the center line of the brace, or if the rivets are pitched too far from the body of the brace, there will be a certain leverage exerted upon the rivets in addition to the direct pull. Fig. 101 shows a brace of incorrect design and Figs. 102 and 103 show braces designed along correct lines.

Other methods of staying, besides the crow foot brace and through stays, consist of gusset stays, and for locomotives and other fire box boilers screwed stay bolts are employed to tie the fire box to the external shell. The holes for these stay bolts are punched or drilled before the fire box is put in place. After it is in and riveted along the lower edge to the foundation ring, or mud ring as it is sometimes called, a continuous thread is tapped in the holes in both the outside plate and the fire sheet y running a long tap through both plates. The steel stay bolt is then screwed through the plates and allowed to project enough at each end to permit of its being riveted cold. Stay bolts are liable to be broken by the unequal expansion of the fire box and outer shell, and a small hole should be drilled in the center of the bolt, from the outer end nearly through to the inner end. Then in case a bolt breaks, steam or water will blow out through the small hole, and the break will be discovered at once. The problem of properly staying the flat crown sheet of a horizontal fire box boiler, especially a locomotive

boiler, is a very difficult one and has taxed the inventive genius of some of the most eminent engineers.

Before the invention of the Belpaire boiler, with its outside or shell plate flat above the fire box, the only method of staying the crown sheet was by the use of cumbersome crown bars or double girders extending across the top of the crown sheet and supported at the ends by special castings that rest on the edges of the side sheets and on the flange of the crown sheet. At intervals of 4 or 5 in. crown bolts are placed, having the head inside the fire box and the nut bearing on a plate on top of the girder. There is also a thimble for each bolt to pass through, between the top of the crown sheet and the girder. These thimbles maintain the proper distance between the crown sheet and girder and allow the water to circulate freely.

The Belpaire fire box dispenses with girders and permits the use of through stays from the top of the flat outside plate through the crown sheet and secured at each end by nuts and copper washers.

For simplicity of construction and great strength the cylindrical form of fire box known as the Morison corrugated furnace has proved to be very successful. This form of fire box was in 1899 applied to a locomotive by Mr. Cornelius Vanderbilt, at the time assistant superintendent of motive power of the New York Central and Hudson River R. R. This furnace was rolled of 34-in. steel, is 59 in. internal diameter and II ft. 24 in. in length. It was tested under an external pressure of 500 lbs. per sq. in. before being placed in the boiler. It is carried at the front end by a row of radial sling stays from the outside plate, and supported at the rear by the back head. Figs. 104 and 105 show respectively a sectional view and an end elevation of