It will appear, by inspection of the duties registered in the preceding table, that the augmentation of the efficiency of the engines has not been the effect of any great or sudden improvement, but has rather resulted from the combination of a great number of small improvements in the details of the operation of these machines. In these improvements more is due to the successful application of practical experience than to any new principles developed by scientific research. Mr. John Taylor, in his "Records of Mining," has traced the successive improvements on which the increased duty of engines depends, and has connected these improvements with their causes in the order of their dates. The following results, abridged from his estimates, may not be uninteresting:— In 1769, soon after the date of the earliest discoveries of Mr. Watt, but before they had come into practical application, Smeaton computed that the average duty of fifteen atmospheric engines, working at Newcastle-on-Tyne, was 5,590,000. The duty of the best of these engines was 7,440,000, and that of the worst 3,220,000. In 1772, Smeaton commenced his improvements on the atmospheric engine, and raised the duty to 9,450,000. In 1776, Watt obtained a duty of 21,600,000. At this time Smeaton acknowledged that Watt's engines gave a duty amounting to double that of his own. In 1778-79, Watt reported a duty of 23,400,000. From 1779 to 1788, Watt introduced the application of expansion, and raised the duty to 26,600,000. [Pg300] In 1798, an engine by Boulton and Watt, erected at Herland, was reported as giving a duty of 27,000,000. This engine, which was probably the best which at that time had ever been erected, attracted the particular attention of Mr. Watt, who, on visiting Cornwall, went to see it, and had many experiments tried with it. It was under the care of Mr. Murdock, the agent of Messrs. Boulton and Watt in Cornwall. When Mr. Watt inspected it he pronounced it perfect, and that further improvement could not be expected. How singular an instance this of the impossibility, even of the most sagacious, to foresee the results of mechanical improvement! In twenty years afterwards the average duty of the best engine was nearly 40,000,000, and in forty years it was above 84,000,000. [Illustration: BOILER MANUFACTORY.] FOOTNOTES: