Dr. Darwin was a man of large and massive person, bearing a rather striking resemblance to his distinguished townsman, Dr. Johnson, in manner, deportment, and force of character. He was full of anecdote, and his conversation was most original and entertaining. He was a very outspoken man, vehemently enunciating theories which some thought original and others dangerous. As he drove through the country in his "sulky," his mind teemed with speculation on all subjects, from zoonomy, botany, and physiology, to physics, æsthetics, and mental philosophy. Though his speculations were not always sound, they were clever and ingenious, and, at all events, they had the effect of setting other minds a-thinking and speculating on science and the methods for its advancement. From his "Loves of the Plants"--afterward so cleverly parodied by George Canning in his "Loves of the Triangles"--it would appear that the doctor even entertained a theory of managing the winds by a little philosophic artifice. His scheme of a steam locomotive was of a more practical character. This idea, like so many others, first occurred to him in his "sulky."
"As I was riding home yesterday," he wrote to his friend Boulton in the year 1765, "I considered the scheme of the fiery chariot, and the longer I contemplated this favorite idea, the more practicable it appeared to me. I shall lay my thoughts before you, crude and undigested though they may appear to be, telling you as well what I thought would not do as what would do, as by those hints you may be led into various trains of thinking upon this subject, and by that means (if any hints can assist your genius, which, without hints, is above all others I am acquainted with) be more likely to improve or disapprove. And as I am quite mad of this scheme, I beg you will not mention it, or show this paper to Wyat or any body.
"These things are required: 1st, a rotary motion; 2d, easily altering its direction to any other direction; 3d, to be accelerated, retarded, destroyed, revived instantly and easily; 4th, the bulk, the weight, and expense of the machine to be as small as possible in proportion to its use."[10]
He then goes on to throw out various suggestions as to the form and arrangement of the machine, the number of wheels on which it was to run, and the mode of applying the power. The text of this letter is illustrated by rough diagrams, showing a vehicle mounted on three wheels, the foremost or guiding wheel being under the control of the driver; but in a subsequent passage he says, "I think four wheels will be better."