In the manufacture of iron, it is of great importance to keep the furnaces continually blown, so that the heat may never be abated by day or night. In the extensive ironworks at Colebrook Dale, several water-wheels were used in the different operations of the manufacture of iron, especially in driving the blowers of the iron furnaces. These wheels were usually driven by the water of a river, but in the summer months the supply became so short that it was insufficient to work them all. Steam engines were accordingly erected to [Pg182] return the water for driving these wheels. This application of the engine as an occasional power for the supply of water-wheels having been found so effectual, returning engines were soon adopted as the permanent and regular means of supplying water-wheels. The first attempt of this kind is recorded to have been made by Mr. Oxley, in 1762, who constructed a machine to draw coals out of a pit at Hartley colliery, in Northumberland. It was originally intended to turn the machine by a continuous circular motion received from the beam of the engine; but that method not being successful, the engine was applied to raise water for a wheel by which the machine was worked. This engine was continued in use for several years, and though it was at length abandoned, on account of its defective construction, it nevertheless established the practicability of using steam power as a means of driving water wheels.[23] (109.) In the year 1777, Mr. John Stewart read a paper before the Royal Society, describing a method for obtaining a continued circular motion for turning all kinds of mills from the reciprocating motion of a steam engine. He proposed to accomplish this by means of two endless chains passing over pulleys, which should be moved upwards and downwards by the motion of the engine, in the manner of a window sash. The joint pins of the links of the two chains worked in teeth at the opposite sides of a cog wheel, to which they imparted a circular motion, first by one chain, and then by the other, acting alternately on opposite sides of the wheel. One chain impelled it during the descent of the piston, and the other during the ascent; but one of these chains always passed over its pulleys so as to produce no effect on one side of the cog wheel, whilst the other chain worked on the opposite side to turn it round. For this purpose each chain was provided with a catch, to prevent its circulating over its pulleys in one direction, but to allow it free motion in the other. The cog wheel thus kept in revolution might be applied to the axis of any mill which the engine was required to work. Thus, if it were applied to a flour-mill, the millstone itself would perform the office of a fly-wheel to regulate the intermission of [Pg183] the power, and in other mills a fly-wheel might be added for this purpose.