This type was especially used in Europe, but on the up stroke there was no work being done, and hence half of the time was lost. This and other difficulties led finally to the adoption of the circular type, whose continuous cut and high speed saved much time and presented many other advantages. A representative example of the circular saw is given in Fig. 241. [Illustration: FIG. 241.--PORTABLE CIRCULAR SAW.] With the increased diameter and peripheral speed of the circular saw, however, a grave difficulty presented itself. The saw would heat at its periphery, and its rim portion expanding without commensurate expansion of the central portion, would cause the saw to crack and fly to pieces under the tremendous centrifugal force. This difficulty is provided for by what is known as “_hammering to tension_,” _i. e._, the saw is hammered to a gradually increasing state of compression from the rim to the center, thus causing an initial expansion or spread of the molecules of metal of the central parts of the saw, which is stored up as an elastic expansive force that accommodates itself to the tension caused by the expansion of the rim, and prevents the unequal and destructive strain, due to the expansion of the rim from the great heat of friction in passing through the log. Mounted upon a portable frame, this machine was put to its great work upon the logs in the forests of America, and for many years this type of sawmill held its sway, and an enormous amount of work was done through its agency. Among its useful accessories were the set-works for adjusting the log holding knees to the position for a new cut, log turners for rotating the log to change the plane of the cut, and the rack and pinion feed, by which the saw carriage was run back and forth. Following the rack and pinion feed came the rope feed, in which a rope wrapped around a drum was carried at its opposite ends over pulleys and back to the opposite ends of the carriage, which was thereby carried back and forth by the forward or backward movement of the drum. [Illustration: FIG. 242.--DIRECT-ACTING STEAM FEED SAWMILL CARRIAGE.]