_Answer._ The most obvious way of increasing the strength of such a seam is to place, or, as it is called, _space_, the rivets further apart, which would leave more metal between the holes, and thus strengthen the seam at its weakest part. But if this is done, it is said that there is difficulty in keeping the seam water-tight, as the plates are then liable to spring apart between the rivets. Another way of increasing its strength is to drill the rivet holes. As already stated, the difference in the strength of the metal left between drilled and punched holes has been shown to be from 10 to 20 per cent. There is also another advantage in drilling the holes for rivets. In punching them, it is necessary to punch each plate separately, and even with the utmost care and skill it is impossible to get the holes to match perfectly. Some of them will overlap each other, as shown in fig. 57, so that when the rivet is set, it will assume somewhat the form shown in fig. 58. There is then danger that those rivets which fill the holes that match each other will be subjected to an undue strain. If, for example, we have five rivet holes, as shown in fig. 59, and only the centre ones correspond with each other, then the rivets in all the other holes will assume somewhat the form shown in fig. 58, and therefore the centre rivet _c_, in fig. 59, which fits the holes accurately, must take the strain of the other four until they draw up “to a bearing.” Under such circumstances, which are not unusual, there will be great danger either of shearing off the rivet _c_, or of starting a fracture in the plates, as indicated by the irregular line _a b_, between the adjoining rivets. It is also obvious that a rivet like the one in fig. 58 will not hold the plates together so well as one which fits more perfectly, as shown in section in fig. 53, and therefore there is more danger of leakage between the plates from badly fitted rivets than from those which fill the holes more perfectly; consequently rivets which fit imperfectly must be placed nearer together than those which are well fitted. It is true that rivets which are set with a riveting machine fill any inaccuracies of the holes more perfectly than those which are set by hand. But even if they are made to fill the holes as shown in fig. 60 they are still not so strong to resist shearing nor so efficient in holding the plates together as they would be if the holes conformed more perfectly to each other. In drilling the holes, the second plate can be drilled from the holes in the first, so that the holes in each will correspond with each other perfectly. The rivets will therefore fit more accurately, and consequently can be spaced further apart, and still keep the plates tight, and thus have more material between the holes, which is the weakest part of the seam. It has been shown that a rivet ¹¹⁄₁₆ inch in diameter has a resistance to shearing of 18,560 pounds. There