But, strange to say, the expedient of driving coal-wagons by the wind was revived in Wales about a century later. On this occasion, Sir Humphry Mackworth, an ingenious coal-miner at Neath, was the projector. Waller, in his "Essay on Mines," published in 1698, takes the opportunity of eulogizing Sir Humphry's "new sailing-wagons, for the cheap carriage of his coal to the water-side, whereby one horse does the work of ten at all times; but when any wind is stirring (which is seldom wanting near the sea), one man and a small sail do the work of twenty."[8] It does not, however, appear that any other coal-owner had the courage to follow Sir Humphry's example, and the sailing-wagon was forgotten until, after the lapse of another century, it was revived by Mr. Edgeworth. The employment of steam-power as a means of land locomotion was the subject of much curious speculation long before any practical attempt was made to carry it into effect. The merit of promulgating the first idea with reference to it probably belongs to no other than the great Sir Isaac Newton. In his "Explanation of the Newtonian Philosophy," written in 1680, he figured a spherical generator, supported on wheels, and provided with a seat for a passenger in front, and a long jet-pipe behind, and stated that "the whole is to be mounted on little wheels, so as to move easily on a horizontal plane, and if the hole, or jet-pipe, be opened, the vapor will rush out violently one way, and the wheels and the ball at the same time will be carried the contrary way." This, it will be observed, was but a modification of the earliest known steam-engine, or OEolopile, of Hero of Alexandria. It is not believed that Sir Isaac Newton ever made any experiment of his proposed method of locomotion, or did more than merely throw out the idea for other minds to work upon. The idea of employing steam in locomotion was revived from time to time, and formed the subject of much curious speculation. About the middle of last century we find Benjamin Franklin, then agent in London for the United Provinces of America, Matthew Boulton, of Birmingham, and Erasmus Darwin, of Lichfield, engaged in a correspondence relative to steam as a motive power. Boulton had made a model of a fire-engine, which he sent to London for Franklin's inspection; and though the original purpose for which the engine had been contrived was the pumping of water, it was believed to be practicable to employ it also as a means of locomotion. Franklin was too much occupied at the time by grave political questions to pursue the subject; but the sanguine and speculative mind of Erasmus Darwin was inflamed by the idea of a "fiery chariot," and he pressed his friend Boulton to prosecute the contrivance of the necessary steam machine.[9]