"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." "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]