Stephenson's idea was that such a road might be made to _float_ upon the bog simply by means of a sufficient extension of the bearing surface. As a ship, or a raft capable of sustaining heavy loads, floated in water, so, in his opinion, might a light road be floated upon a bog which was of considerably greater consistency than water. Long before the railway was thought of, Mr. Roscoe had adopted the remarkable expedient of fitting his plow-horses with flat wooden soles or pattens, to enable them to walk upon the Moss land which he had brought into cultivation. These pattens were fitted on by means of a screw apparatus, which met in front of the foot and was easily fastened. The mode by which these pattens served to sustain the horse is capable of easy explanation, and it will be observed that the _rationale_ alike explains the floating of a railway. The foot of an ordinary farm-horse presents a base of about five inches diameter, but if this base be enlarged to seven inches--the circles being to each other as the squares of the diameters--it will be found that, by this slight enlargement of the base, a circle of nearly double the area has been secured, and consequently the pressure of the foot upon every unit of ground on which the horse stands has been reduced one half. In fact, this contrivance has an effect tantamount to setting the horse upon eight feet instead of four. Apply the same reasoning to the ponderous locomotive, and it will be found that even such a machine may be made to stand upon a bog by means of a similar extension of the bearing surface. Suppose the engine to be twenty feet long and five feet wide, thus covering a surface of a hundred square feet, and, provided the bearing has been extended by means of cross sleepers supported upon a matting of heath and branches of trees covered with a few inches of gravel, the pressure of an engine of twenty tons will be only equal to about three pounds per inch over the whole surface on which it stands. Such was George Stephenson's idea in contriving his floating road--something like an elongated raft--across the Moss; and we shall see that he steadily kept it in view in carrying the work into execution.