design is probably to be credited to Bull originally; but Trevithick built some of these engines, and is supposed to have greatly improved them while working with Edward Bull, the son of the inventor, William Bull. One of these engines was erected by them at the Herland Mine, Cornwall, in 1798, which had a steam-cylinder 60 inches in diameter, and was built on the plan just described. Another of the contemporaries of James Watt was a clergyman, EDWARD CARTWRIGHT, the distinguished inventor of the power-loom, and of the first machine ever used in combing wool, who revived Watt's plan of surface-condensation in a somewhat modified form. Watt had made a "pipe-condenser," similar in plan to those now often used, but had simply immersed it in a tank of water, instead of in a constantly-flowing stream. Cartwright proposed to use two concentric cylinders or spheres, between which the steam entered when exhausted from the cylinder of the engine, and was condensed by contact with the metal surfaces. Cold water within the smaller and surrounding the exterior vessel kept the metal cold, and absorbed the heat discharged by the condensing vapor. Cartwright's engine is best described in the _Philosophical Magazine_ of June, 1798, from which the accompanying sketch is copied. [Illustration: FIG. 39.--Cartwright's Engine, 1798.] The object of the inventor is stated to have been to remedy the defects of the Watt engine--imperfect vacuum, friction, and complication. In the figure, the steam-cylinder takes steam through the pipe, _B_. The piston, _R_, has a rod extending downward to the smaller pump-piston, _G_, and upward to the cross-head, which, in turn, drives the cranks above, by means of connecting-rods. The shafts thus turned are connected by a pair of gears, _M L_, of which one drives a pinion on the shaft of the fly-wheel. _D_ is the exhaust-pipe leading to the condenser, _F_; and the pump, _G_, removes the air and water of condensation, forcing it into the hot-well, _H_, whence it is returned to the boiler through the pipe, _I_. A float in _H_ adjusts an air-valve, so as to keep a supply of air in the chamber, to serve as a cushion and to make an air-chamber of the reservoir, and permits the excess to escape. The large tank contains the water supplied for condensing the steam. The piston, _R_, is made of metal, and is packed with two sets of cut metal rings, forced out against the sides of the cylinder by steel springs, the rings being cut at three points in the circumference, and kept in place by the springs. The arrangement of the two cranks, with their shafts and gears, is intended to supersede Watt's plan for securing a perfectly rectilinear movement of the head of the piston-rod, without friction.