Note 101 — Chapter XVIII

The hydraulic presses were of an extraordinary character. The cylinders of those first constructed were of wrought iron (cast iron being found altogether useless), not less than 8 inches thick. They were tested by being subjected to an internal pressure of 3 or 3-1/2 tons to the circular inch. The pressure was such that it squeezed the fibres of the iron together; so that, after a few tests of this character, the piston, which at first fitted it quite closely, was found considerably too small. "A new piston," says Mr. Clark, "was then made to suit the enlarged cylinder; and a farther enlargement occurring again and again with subsequent use, the new pistons became as formidable an obstacle as the cylinders. The wrought-iron cylinder was on the point of being abandoned, when Mr. Amos (the iron manufacturer), having carefully gauged the cylinder inside and out, found to his surprise that, although the internal diameter had increased considerably, the external diameter had retained precisely its original dimensions. He consequently persevered in the construction of new pistons, and ultimately found that the cylinder enlarged no longer, and to this day it continues in constant use. Layer after layer having attained additional permanent set, sufficient material was at length brought into play, with sufficient tenacity to withstand the pressure; and thus an obstacle, apparently insurmountable, and which threatened at one time to render much valuable machinery useless, was entirely overcome. The workman may be excused for calling the stretched cylinder stronger than the new one, though it is only stronger as regards the amount of its yielding to a given force."—Clark, vol. i., p. 306. The hydraulic presses used in raising the tubes of the Britannia Bridge, it may be remembered, were afterward used in starting the Great Eastern from her berth on the shore at Milwall, where she had been built.