The second cause of the reduction of strength is owing to the injury sustained by the plates during the process of drilling and punching. The knowledge existing regarding this subject is not very accurate, although numerous experiments have been made to determine the exact amount of weakening caused by punching plates. It is, however, certain that in many cases the strength of the metal _left_ between the holes of boiler plates is reduced from 10 to 30 per cent. by the process of punching. It is probable, however, that soft ductile metal is injured less than that which is harder and more brittle. Some kinds of steel plates are especially liable to injury from punching. It is also probable that the condition of the punch, and the proportions of the die used with it, have much to do with its effect upon the metal The third cause of weakness is owing to the fact that if one or more holes are made in a plate of any material, and it is then subjected to a tensile strain, the strain, instead of being equally distributed through the section left between the holes, will be greatest in that part of the metal nearest them. This can be illustrated by taking a band of india-rubber, fig. 54, and cutting a round hole in it to represent a rivet hole. If we draw two parallel lines, _A B_, across the band and then stretch it, the lines, instead of remaining parallel when the band is stretched, will separate most next to the hole, as shown in fig. 55, indicating that the fibres of the rubber nearest the hole are strained most. A similar effect takes place when a plate of iron is stretched, so that a fracture is liable to begin next to the hole, after which the plate will be broken as it were in detail. [Illustration: _Fig. 54._] [Illustration: _Fig. 55._] QUESTION 103. _How may a boiler seam like that shown in fig. 53 break?_ _Answer._ It may break in three different ways: 1. By the plate tearing between the rivet holes on the line _a b_. 2. By the rivets shearing off. 3. By the plate in front of the rivets crushing, as shown in fig. 56. [Illustration: _Fig. 56._] QUESTION 104. _How can the strength of a boiler seam be calculated at each of these three points?_