James Watt's friend and correspondent, Dr. Small, of Birmingham, when he heard of Moore's intended project, wrote to the Glasgow inventor with the object of stimulating him to perfect his steam-engine, then in hand, and urging him to apply it, among other things, to purposes of locomotion. "I hope soon," said Small, "to travel in a fiery chariot of your invention." Watt replied to the effect that "if Linen-draper Moore does not use my engines to drive his carriages, he can't drive them by steam. If he does, I will stop them." But Watt was still a long way from perfecting his invention. The steam-engine capable of driving carriages was a problem that remained to be solved, and it was a problem to the solution of which Watt never fairly applied himself. It was enough for him to accomplish the great work of perfecting his condensed engine, and with that he rested content. But Watt continued to be so strongly urged by those about him to apply steam-power to purposes of locomotion that, in his comprehensive patent of the 24th of August, 1784, he included an arrangement with that object. From his specification we learn that he proposed a cylindrical or globular boiler, protected outside by wood strongly hooped together, with a furnace inside entirely surrounded by the water to be heated except at the ends. Two cylinders working alternately were to be employed, and the pistons working within them were to be moved by the elastic force of the steam; "and after it has performed its office," he says, "I discharge it into the atmosphere by a proper regulating valve, or I discharge it into a condensing vessel made air-tight, and formed of thin plates and pipes of metal, having their outsides exposed to the wind;" the object of this latter arrangement being to economize the water, which would otherwise be lost. The power was to be communicated by a rotative motion (of the nature of the "sun and planet" arrangement) to the axle of one or more of the wheels of the carriage, or to another axis connected with the axle by means of toothed wheels; and in other cases he proposed, instead of the rotative machinery, to employ "toothed racks, or sectors of circles, worked with reciprocating motion by the engines, and acting upon ratched wheels fixed on the axles of the carriage." To drive a carriage containing two persons would, he estimated, require an engine with a cylinder 7 in. in diameter, making sixty strokes per minute of 1 ft. each, and so constructed as to act both on the ascent and descent of the piston; and, finally, the elastic force of the steam in the boiler must be such as to be occasionally equal to supporting a pillar of mercury 30 in. high.