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."
"Let there be two cylinders," he proceeds. "Suppose one piston up, and the vacuum made under it by the jet d'eau froid. That piston can not 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 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 can not fail of success! Eureka!
"The cocks of the cold water may be moved by the great work, but the steam cocks must be managed by the hand of the charioteer, who also directs the rudder-wheel. [Then follow his rough diagrams.] The central wheel ought to have been under the rollers, so as it may be out of the way of the boiler."[11]
After farther explaining himself, he goes on to say:
"If you could learn the expense of coals to a common fire-engine and the weight of water it draws, some certain estimate may be made if such a scheme as this would answer. Pray don't show Wyat this scheme, for if you think it feasible and will send me a critique upon it, I will certainly, if I can get somebody to bear half the expense with me, endeavor to build a fiery chariot, and, if it answers, get a patent. If you choose to be partner with me in the profit, and expense, and trouble, let me know, as I am determined to execute it if you approve of it.
"Please to remember the pulses of the common fire-engines, and say in what manner the piston is so made as to keep out the air in its motion. By what way is the jet d'eau froid let out of the cylinder? How full of water is the boiler? How is it supplied, and what is the quantity of its waste of water?"[12]
It will be observed from these remarks that the doctor's notions were of the crudest sort, and, as he obviously contemplated but a modification of the Newcomen engine, then chiefly employed in pumping water from mines, the action of which was slow, clumsy, and expensive, the steam being condensed by injection of cold water, it is clear that, even though Boulton had taken up and prosecuted Darwin's idea, it could not have issued in a practicable or economical working locomotive.
But, although Darwin himself—his time engrossed by his increasing medical practice—proceeded no farther with his scheme of a "fiery chariot," he succeeded in inflaming the mind of his young friend, Richard Lovell Edgeworth, who had settled for a time in his neighborhood, and induced him to direct his attention to the introduction of improved means of locomotion by steam. In a letter written by Dr. Small to Watt in 1768, we find him describing Edgeworth as "a gentleman of fortune, young, mechanical, and indefatigable, who has taken a resolution to move land and water carriages by steam, and has made considerable progress in the short space of time that he has devoted to the study."
One of the first-fruits of Edgeworth's investigations was his paper "On Railroads," which he read before the Society of Arts in 1768, and for which he was awarded the society's gold medal. He there proposed that four iron railroads be laid down on one of the great roads out of London; two for carts and wagons, and two for light carriages and stage-coaches. The post-chaises and gentlemen's carriages might, he thought, be made to go at eight miles an hour, and the stage-coaches at six miles an hour, drawn by a single horse. He urged that such a method of transport would be attended with great economy of power and consequent cheapness. Many years later, in 1802, he published his views on the same subject in a more matured form. By that time Watt's steam-engine had come into general use, and he suggested that small stationary engines should be fixed along his proposed railroad, and made, by means of circulating chains, to draw the carriages along with a great diminution of horse labor and expense.
It is creditable to Mr. Edgeworth's forethought that both the models proposed by him have since been adopted. Horse-traction of carriages on railways is now in general use in the towns of the United States; and omnibuses on the same principle regularly ply between the Place de la Concorde at Paris and St. Cloud, both being found highly convenient for the public, and profitable to the proprietors. The system of working railways by fixed engines was also regularly employed on some lines in the infancy of the railway system, though it has since fallen into disuse, in consequence of the increased power given to the modern locomotive, which enables it to surmount gradients formerly considered impracticable.