A feature peculiar to the boiler and plating shops is the enormous number of rivet holes which have to be made, and of rivets to be inserted. These requirements are reflected in machine design. To punch or drill holes singly is too slow a process in the best practice, and so machines are made for producing many holes simultaneously. Besides this, the different sections of boilers are drilled in machines of different types, some for shells, some for furnaces, some peculiar to the shells or furnaces of one type of boilers, others to those of another type only. And generally now these machines not only drill, but can also be adjusted to drill to exact pitch, the necessity thus being avoided of marking out the holes as guides to the drills. Hand-riveting has mostly been displaced by hydraulic and pneumatic machines, with resulting great saving in cost, and the advantage of more trustworthy and uniform results. For boiler work, machines are mostly of fixed type; for bridge and girder work they are portable, being slung from chains and provided with pressure water or compressed air by systems of flexible pipes. Welding fills a large place in boiler work, but it is that of the edges of plates chiefly, predominating over that of the bars and rods of the smithy. The edges to be united are thin and long, so that short lengths have to be done in succession at successive "heats." Much of this is hand work, and "gluts" or insertion pieces are generally preferred to overlapping joints. But in large shops, steam-driven power hammers are used for closing the welds. Parts that are commonly welded are the furnace flues, the conical cross-tubes and angle rings. Another aspect of the work of these departments is the immense proportions of the modern machine tools used. This development is due in great degree to the substitution of steel for iron. The steel shell-plates of the largest boilers are 1-1/2 in. thick, and these have to be bent into cylindrical forms. In the old days of iron boilers the capacity of rolls never exceeded about 3/4 in. plate. Often, alternatively to rolling, these thick plates are bent by squeezing them in successive sections between huge blocks operated by hydraulic pressure acting on toggle levers. And other machines besides the rolls are made more massive than formerly to deal with the immense plates of modern marine boilers.