From this it is seen that the strength of the seam with drilled plates is 60 per cent. of that of the solid plates, or it is about 18¹⁄₂ per cent. stronger than that made with plates having punched holes and the rivets nearer together. It should be noted that a great part of the superiority of the seams made with drilled holes is due to the superior accuracy of the work done in that way, which makes it possible to use larger rivets spaced further apart. It is probable that with the use of some recently designed machines, intended to produce greater accuracy in punching rivet holes, part of the above advantage may be realized with that kind of work. The greatest distance that rivets may be spaced apart without incurring danger of leakage between the plates must, however, be determined more by practical than theoretical considerations. It is certain, however, that rivets may be spaced much further apart than they are in ordinary practice, and the seams still be kept tight, if the work is done with sufficient accuracy and care. QUESTION 106. _What other methods are there of making boiler seams which are stronger than those which have been described?_ _Answer._ In this country two rows of rivets are used and also what is called a “_welt_,” or _covering-strip_, the latter with both single and double-riveted seams. [Illustration: _Fig. 62._] QUESTION 107. _How are the rivets arranged when two rows are used?_ _Answer._ They are often placed just behind each other as shown in fig. 62, which is called _chain-riveting_. Such an arrangement of rivets obviously adds nothing to the strength of the seam, because its weakest part, as has already been shown, is the section of the plates through the rivet-holes, and this is not at all strengthened by adding another row of rivets, because the plates are just as liable to break through the rivet-holes with two rows of rivets as they would be with one. [Illustration: _Fig. 63._]