Want of water-power was one of the great defects of Soho as a manufacturing establishment, and for a long time Boulton struggled with the difficulty. The severe summer droughts obliged him to connect a horse-mill with the water-wheel. From six to ten horses were employed as an auxiliary power, at an expense of from five to eight guineas a week. But this expedient, though costly, was found very inconvenient. Boulton next thought of erecting a pumping-engine after Savery or Newcomen’s construction, for the purpose of raising the water from the mill-stream and returning it back into the reservoir--thereby maintaining a head of water sufficient to supply the water-wheel and keep the mill in regular work. “The enormous expense of the horse-power,” he wrote to a friend, “put me upon thinking of turning the mill by fire, and I made many fruitless experiments on the subject.” In 1766 we find him engaged in a correspondence with the distinguished Benjamin Franklin as to steam power. Eight years before, Franklin had visited Boulton at Birmingham and made his acquaintance. They were mutually pleased with each other, and continued to correspond during Franklin’s stay in England, exchanging their views on magnetism, electricity, and other subjects.[110] When Boulton began to study the fire-engine with a view to its improvement, Franklin was one of the first whom he consulted. Writing him on the 22nd February, 1766, he said,-- “My engagements since Christmas have not permitted me to make any further progress with my fire-engine; but, as the thirsty season is approaching apace, necessity will oblige me to set about it in good earnest. Query,--Which of the steam-valves do you like best? Is it better to introduce the jet of cold water at the bottom of the receiver, or at the top? Each has its advantages and disadvantages. My thoughts about the secondary or mechanical contrivances of the engine are too numerous to trouble you with in this letter, and yet I have not been lucky enough to hit upon any that are objectionless. I therefore beg, if any thought occurs to your fertile genius which you think may be useful, or preserve me from error in the execution of this engine, you’ll be so kind as to communicate it to me, and you’ll very greatly oblige me.”