“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.
The model duly arrived at Soho, and we find Boulton shortly after occupied in making experiments with it, the results of which are duly entered in his note-books. Dr. Erasmus Darwin, with whom he was on very intimate terms, wrote him from Lichfield, inquiring what Franklin thought of the model and what suggestions he had made for its improvement. “Your model of a steam-engine, I am told,” said he, “has gained so much approbation in London, that I cannot but congratulate you on the mechanical fame you have acquired by it, which, assure yourself, is as great a pleasure to me as it could possibly be to yourself.”[112] Another letter of Darwin to Boulton is preserved, without date, but apparently written earlier than the preceding, in which the Doctor lays before the mechanical philosopher the scheme of “a fiery chariot” which he had conceived,--in other words, of a locomotive steam-carriage. He proposed to apply an engine with a pair of cylinders working alternately, to drive the proposed vehicle;[113] and he sent Boulton some rough diagrams illustrative of his views, which he begged might be kept a profound secret, as it was his intention, if Boulton approved of his plan and would join him as a partner, to endeavour to build a model engine, and, if it answered, to take out a joint patent for it. But Dr. Darwin’s scheme was too crude to be capable of being embodied in a working model; and nothing more was heard of his fiery chariot.