This seam would then have a strength of 72 per cent. of the solid plates, or be 42¹⁄₄ per cent. stronger than the ordinary riveted seam with punched holes. It is important to observe that an increase of strength results from the use of larger rivets spaced farther apart than is usual in ordinary practice, and that this is possible only with the best and most accurate workmanship. [Illustration: _Fig. 64._] QUESTION 108. _What is the form of construction of boiler seams made with a welt or covering-strip?_ _Answer._ The plates (_a_, _b_, fig. 64) are lapped over each other as for an ordinary seam. Another plate, _c_, about nine inches wide, is then placed on the inside of the seam and bent so as to conform to the lap of the two plates. The rivets _r_, whether a double or single row, pass through all three plates, and two more rows of rivets are put next to the edges of the covering plate, _c_. It is plain that the strength of the seam, _r_, is increased up to a certain point by an amount just equal to that of the rivets in the edges of the covering plate. If, however, these are placed too close together, the plates _a_ and _b_ will be weaker through the outside rows of rivets than the seam is through either of the outside ones and the middle one taken together. If, for example, we take a single-riveted seam, like that shown in fig. 53, whose strength is only a little more than half that of the solid plate, and should add to it a covering plate, as shown in fig. 64, and then space the rivets in the edges of the covering plate the same distance apart as in the middle seam, then obviously the plates would be just as liable to break through the outer rows of holes as through the center row before the covering plate was added. If, however, the holes in the two outside plates are spaced at say twice the distance apart, or 3³⁄₄ inches, then the only way the seam can break through the outer rows of holes is by shearing the rivets, because the plates between the holes are then stronger than the rivets. But before these rivets can be sheared, the centre seam must give way. Thus the strength of such a seam is equal to THE SUM OF THE STRENGTH AT THE WEAKEST POINTS OF THE MIDDLE AND THE OUTSIDE SEAMS. The strength of the plates between the holes of the outside rows of rivets must, however, be as great as the sum referred to, otherwise the seam will be the weakest at that point, and the failure will occur there. The rivets in the outside rows should be spaced at least twice as far apart as those in the middle seam. The number of rivets to resist shearing will then be 50 per cent. greater, so that the strength of a seam like that shown in fig. 53, with a covering plate added, will be as follows: