The blows struck by the hammers were very effective. The cams had a throw of 1¹⁄₄ inches, but they threw the hammers back against the springs 1¹⁄₄ inches further, making their fall 2¹⁄₂ inches. This I ascertained by holding a piece of thin board edgeways between the upper end of a hammer and the cross-bar at the back, when the hammer crushed it up to this height. We never ran over the stone with the points but once. They made everything before them fly. On the other hand, the droves merely dusted the surface, to take out the marks of the tooth-chisels. All surplus force in the blow was received on the 6-inch cross-bar. The tools stood motionless unless pushed back by the stone, when they received a sufficient portion of the blow to drive them forward to their position. The feed motion was powerful, being imparted by a worm engaging in a worm-wheel 24 inches in diameter, while the run back was swift, quite 100 feet in a minute. The sides of the steel tool-holders, rubbing against each other, became after a while badly abraded. I was obliged to plane them off and dovetail thin strips of hardened steel into them. These prevented any further trouble. The sides of the end tool-holders, however, which rubbed against the cast-iron side-bars, I observed, were polished without sensible wear. This was a very important observation. These surfaces all rubbed together dry. The pressure was only the side thrust, which was very trifling. Under these conditions the molecules of the same material interlocked, while those of the different materials did not. These two materials were, however, extremely different in their constituent features. Perhaps this point of freedom of _some_ different materials from interlocking was still better illustrated by the set-screws, where this difference of molecular structure did not exist in the same degree. These were made of Ulster iron, a superior quality of American iron then largely used in New York City for bolts. They were ⁵⁄₈-inch screws, and were also used dry, no oil being allowed anywhere over the stones. Each tool-holder contained three of these set-screws. The outside ones were tightened and loosened sixty times every day. The middle ones, where only the points were used, were tightened and loosened twenty times every day and at other times stood loose in their threads. The tool-holders being massive, and the blows of the hammers also coming on the leather cushion, there was no vibration. At the end of the two years’ running the outer bolts were all perfect fits. The middle ones were loose, but still held the tools perfectly.