principle; and it appeared doubtful, from the trials which had been made of it, whether it would ever prove an economical working engine. Watt then returned to Birmingham, to proceed with the completion of his rotary motion. Boulton kept urging that the field for pumping-engines was limited, that their Cornish prospects were still gloomy, and that they must very soon look out for new fields. One of his schemes was the applying of the steam-engine to the winding of coals. “A hundred engines at 100_l._ a year each,” he said, “would be a better thing than all Cornwall.” But the best field of all, he still held, was mills. “Let us remember,” said he, “the Birmingham motto, to ‘strike while the iron is hot.’” Watt, as usual, was not so sanguine as his partner, and rather doubtful of the profit to be derived from this source. From a correspondence between him and Mr. William Wyatt, of London, on the subject, we find him discouraging the scheme of applying steam-engines to drive corn-mills; on which Boulton wrote to Wyatt,-- “You have had a correspondence with my friend Watt, but I know not the particulars.... You must make allowance in what Mr. Watt says ... he _under_ values the merits of his own works.... _I_ will take all risks in erecting an engine for a corn-mill.... I think I can safely say our engine will grind _four_ times the quantity of corn per bushel of coal compared with any engine hitherto erected.”[251] [251] Boulton to Wyatt, 16th December, 1782. About the same time we find Boulton writing to Watt,-- “You seem to be fearful that mills will not answer, and that you cannot make Reynolds’s amount to more than 20_l._ a year. For my part, I think that mills, though trifles in comparison with Cornish engines, present a field that is boundless, and that will be more permanent than these transient mines, and more satisfactory than these inveterate, ungenerous, and envious miners and mine lords. As to the trouble of small engines, I would curtail it by making a pattern card of them (which may be done in the course of next year), and confine ourselves to those sorts and sizes until our convenience admits of more.”[252] [252] Boulton to Watt, 7th December, 1782.