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. [112] Darwin to Boulton, March 11, 1766. Boulton MSS. [113] The following passage occurs in his letter:--“Suppose one piston up, and the vacuum made under it by the jet d’eau froid. That piston cannot yet descend, because the cock is not yet opened which admits the steam into its antagonist cylinder. Hence the two pistons are in equilibrio, being either of them pressed by the atmosphere. Then, I say, if the cock which admits the steam into the antagonist cylinder be opened gradually and not with a jerk, that the first mentioned [piston in the] cylinder will descend gradually and yet not less forcibly. Hence by the management of the steam cocks the motion may be accelerated, retarded, destroyed, revived, instantly and easily. And if this answers in practice as it does in theory, the machine cannot fail of success! Eureka!”