Though Watt repeatedly expressed his intention of constructing a model locomotive after his specification, it does not appear that he ever carried it out. He was too much engrossed with other work; and, besides, he never entertained very sanguine views as to the practicability of road locomotion by steam. He continued, however, to discuss the subject with his partner Boulton, and from his letters we gather that his mind continued undetermined as to the best plan to be pursued. Only four days after the date of the above specification (_i.e._, on the 28th of August, 1784) we find him communicating his views on the subject to Boulton at great length, and explaining his ideas as to how the proposed object might best be accomplished. He first addressed himself to the point of whether 80 lbs. was a sufficient power to move a post-chaise on a tolerably good and level road at four miles an hour; secondly, whether 8 ft. of boiler surface exposed to the fire would be sufficient to evaporate a cube foot of water per hour without much waste of fuel; thirdly, whether it would require steam of more than eleven and a half times atmospheric density to cause the engine to exert a power equal to 6 lbs. on the inch. "I think," he observed, "the cylinder must either be made larger or make more than sixty strokes per minute. As to working gear, stopping and backing, with steering the carriage, I think these things perfectly manageable." "My original ideas on the subject," he continued, "were prior to my invention of these improved engines, or before the crank, or any other of the rotative motions were thought of. My plan then was to have two inverted cylinders, with toothed racks instead of piston-rods, which were to be applied to two ratchet-wheels on the axle-tree, and to act alternately; and I am partly of opinion that this method might be applied with advantage yet, because it needs no fly and has some other conveniences. From what I have said, and from much more which a little reflection will suggest to you, you will see that without several circumstances turn out more favorable than has been stated, the machine will be clumsy and defective, and that it will cost much time to bring it to any tolerable degree of perfection, and that for me to interrupt the career of our business would be imprudent; I even grudge the time I have taken to make these comments on it. There is, however, another way in which much mechanism might be saved if it be in itself practicable, which is to apply to it one of the self-moving rotatives, which has no regulators, but turns like a mill-wheel by the constant influx and efflux of steam; but this would not abridge the size of the boiler, and I am not sure that such engines are practicable."