[110] On the 22nd May, 1765, Franklin writes Boulton,--“Mr. Baskerville informs me that you have lately had a considerable addition to your fortune, on which I sincerely congratulate you. I beg leave to introduce my friend Doctor Small to your acquaintance, and to recommend him to your civilities. I would not take this freedom, if I were not sure it would be agreeable to you; and that you will thank me for adding to the number of those who from their knowledge of you must respect you, one who is both an ingenious philosopher and a most worthy honest man. If anything new in magnetism or electricity, or any other branch of natural knowledge, has occurred to your fruitful genius since I last had the pleasure of seeing you, you will by communicating it greatly oblige me.” From a subsequent letter it appears that Boulton, like Watt--who was about the same time occupied with his invention at Glasgow--had a model constructed for experimental purposes, and that this model was now with Franklin in London; for we find Boulton requesting the latter to “order a porter to nail up the model in the box again and take it to the Birmingham carrier at the Bell Inn, Smithfield.” After a silence of about a month Franklin replied,-- “You will, I trust, excuse my so long omitting to answer your kind letter, when you consider the excessive hurry and anxiety I have been engaged in with our American affairs.... I know not which of the valves to give the preference to, nor whether it is best to introduce your jet of cold water above or below. Experiments will best decide in such cases. I would only repeat to you the hint I gave, of fixing your grate in such a manner as to burn all your smoke. I think a great deal of fuel will then be saved, for two reasons. One, that smoke is fuel, and is wasted when it escapes uninflamed. The other, that it forms a sooty crust on the bottom of the boiler, which crust not being a good conductor of heat, and preventing flame and hot air coming into immediate contact with the vessel, lessens their effect in giving heat to the water. All that is necessary is, to make the smoke of fresh coals pass descending through those that are already thoroughly ignited. I sent the model last week, with your papers in it, which I hope got safe to hand.”[111] [111] Franklin to Boulton, March 19, 1766. Boulton MSS.