In riveted connections the various parts of the bridge are fastened at all junctions by overlapping the plates of iron or steel and inserting rivets into holes punched through all the plates to be connected. The rivets are so spaced as to insure the best result as to strength. The pieces of metal are brought together, either in the shop or at the structure during erection, and the rivets, which are round pieces of metal with a head formed on one end, are heated and inserted from one side, being made long enough to project sufficiently to give the proper amount of metal for forming the other head. This is done while the rivet is still hot, either by hammering or by the application of a riveting machine, operated by steam or hydraulic pressure. Ingenious portable machines are now manufactured which are hung from the structure during erection and connected by flexible hose with the steam power, by the use of which the rivet heads can be formed in place with great celerity. The connections of plates by rivets of proper dimensions and properly spaced give great strength and stiffness to such joints. In pin connections the members of a structure are assembled at points of junction and a large iron or steel pin inserted in a pin-hole running through all the members. This pin is made of such diameter as to withstand and properly transmit all the strains brought upon it. Joints made with such pin connections have flexibility, and the strains and stresses can be calculated with great precision. Eye-bars are forged pieces of iron or steel, generally flat, and enlarged at the ends so as to give a proper amount of metal around the pin-hole or eye, formed in those ends. Structures connected by pins at their principal junctions have, of course, many parts in which riveting must be used. The elements which are distinctively American in our railway bridges are the concentration of material in few members and the use of eye-bars and pin connections in place of riveted connections. The riveted methods are, however, largely used in connection with the American forms of truss construction. [Illustration: Truss Bridge of the Northern Pacific Railway over the Missouri River at Bismarck, Dak.--Testing the central span.]