is therefore no advantage in spacing such rivets further apart than 1¹³⁄₁₆ from centre to centre, because the metal left between drilled holes that distance apart would be slightly stronger than the rivets. If therefore the rivets are placed further apart, their diameter must be increased. There is, however, a limit beyond which the diameters of rivets cannot be increased with advantage, because if we increase their diameters, their sectional area to resist shearing is increased in proportion _to the square of the diameter_, whereas the section of metal in the plate to resist crushing is increased only _in proportion to the diameter_. This will be apparent if we compare a rivet ¹⁄₂ inch with one 1 inch in diameter. The first has a sectional area of .1963 inch, the other .7854 inch, or _four_ times that of the first one. Now the area which resists the crushing strain of the rivets is increased only in proportion to their diameters, or is _twice_ as much for the one as for the other. If, therefore, we increase the diameters of the rivets, we very soon reach a point at which the plate has less strength to resist crushing than the rivet has to resist shearing. The diameter of rivet which will give just the same resistance to both strains varies with the thickness of the plates; with ³⁄₈ inch plates a ⁷⁄₈ rivet will have a resistance to shearing of 30,065 pounds and the plate in front of it a resistance to crushing of 29,530 pounds. A ⁷⁄₈ rivet is, therefore, the largest size which can be used to advantage in ³⁄₈ plates. If now we were to space such rivets so far apart that the metal left between the holes would have a strength just equal to that of the rivets, we would have the strongest possible seam that can be made with a single row of rivets. This distance would be 1³⁄₄ inches between the edges of the rivets, or 2⁵⁄₈ from center to center, as shown in fig. 61. The following table will show the strength of such a seam composed of four rivets, and two plates 10¹⁄₂ inches wide,[31] with drilled holes: Plates through rivet holes (tearing) 118,125 lbs. Rivets (shearing) 120,260 lbs. Plates in front of rivets (crushing) 118,125 lbs. Solid plates (tearing) 196,875 lbs. [31] It has been necessary to take for an illustration, plates of a different width from the preceding example, in order to get an even number of spaces between the rivets in each case. [Illustration: _Fig. 61._]