_Answer._ There is great variation in the tensile strength[27] of rolled iron boiler plate, but that of good plate will average about 50,000 pounds per square inch, if the strain is applied in the direction of the “grain” or the fibres of the iron[28], and about ten per cent. less if the strain is applied crosswise of the grain. It has, however, been found by experiment that when a tensile strain is applied to a bar of iron or other material, it is stretched a certain amount in proportion to the length of the bar and to the degree of strain to which it is subjected. It is found that if this strain does not exceed about one-fifth of that which would break the bar, it will recover its original length, or will contract after being stretched, when the strain is removed. The greatest strain which any material will bear without being permanently stretched is called its _limit of elasticity_, and so long as this is not exceeded no appreciable permanent elongation or “set” will be given to iron by any number of applications of such strains or loads. If, however, the limit of elasticity is exceeded, the metal will be permanently elongated, and this elongation will be increased by repeated applications of the strain until finally the bar will break. At the same time the character of the metal will be altered by the repeated application of strains greater than its elastic limit, and it will become brittle and less able to resist a sudden strain, and will ultimately break short off. It is therefore unsafe to subject iron, or in fact any other material, to strains greater than its elastic limit. This limit for iron boiler plates may be taken at about one-fifth its breaking, or, as it is called, _ultimate_ strength. It should be remembered, however, in this connection, that it often happens that the steam pressure is not the greatest force the boiler must withstand, as sudden or unequal expansion and contraction are probably more destructive, to locomotive boilers especially, than the pressure of the steam. [27] A force exerted to pull any material apart is called a _tensile strain_, and if exerted to compress it is called a _compressive strain_. [28] It should be explained that in the process of manufacturing iron by rolling, the iron is stretched out into fibres in the direction in which it passes between the rolls. QUESTION 101. _How are the plates of boilers fastened together?_