Plates through outside rows of rivet-holes (tearing) 91,880 lbs. Rivets in one outside and middle row (shearing) 111,360 lbs. Plates in front of rivets (crushing) 139,218 lbs. Solid plates (tearing) 140,625 lbs. An ordinary single-riveted seam with punched holes, with a welt or covering plate added, would thus have a strength equal to 65.3 per cent. of the solid plate, or be 29 per cent. stronger than the seam without the covering plate. It is probable, however, that the injury to the plate from punching the outside rows of holes which are further apart is not so great as it is when they are punched nearer together and to the edge, so that the strength is somewhat greater than our estimate. The relative strength of the different forms of seams described in percentage of the strength of the solid plate is then as follows: Percentage of strength compared with solid plate. Single-riveted seam, punched holes ¹¹⁄₁₆ rivets 50.6 Single-riveted seam, drilled holes ⁷⁄₈ rivets 60. Double-riveted seam, drilled zigzag holes ³⁄₄ rivets 70. Double-riveted seam, drilled zigzag holes ⁷⁄₈ rivets 72. Single riveted seam, punched holes with covering plate 65.3 QUESTION 109. _Are there any other forms of boiler seams used?_ _Answer._ In Europe what are called _butt-joints_ are used to some extent. In these the ends of the two plates abut against each other, with a covering strip on one or both sides. This form of joint is, however, not used in this country, and therefore its peculiarities will not be described. QUESTION 110. _How are the seams of boilers made water-tight?_ _Answer._ By what is called _caulking_. That is, by the use of a blunt instrument somewhat resembling a chisel, the end of which is placed against one or both of the edges of the plates _c_, _d_, fig. 53, which are then riveted down by blows of a hammer, somewhat as the joints of a ship are made tight. Before the edges, which are called the _caulking edges_, of the plates are made tight in this way, they are cut or trimmed off with a chisel. In this process the plate under the edge is often injured seriously by the carelessness of workmen, who sometimes allow the chisel to cut a groove in the plate under the edge, thus weakening it at a point where the greatest strength is needed. There is also danger of forcing the plates apart in the manner shown in fig. 65, if it is done carelessly and with a heavy hammer. [Illustration: _Fig. 65._] QUESTION 111. _How are the flat ends of the boiler strengthened?_