The great principle of exchange between one part of this empire and another part, which has ceased to be an affair of restrictions and jealousies, has covered the island with good roads, with canals, and finally with railways. The railway and the steam-carriage have carried the principle of diminishing the price of conveyance, and therefore of commodities, by machinery, to an extent which makes all other illustrations almost unnecessary. A road with a waggon moving on it is a mechanical combination; a canal, with its locks, and towing-paths, and boats gliding along almost without effort, is a higher mechanical combination; a railway, with its locomotive engine, and carriage after carriage dragged along at the rate of thirty or forty miles an hour, is the highest of such mechanical combinations. The force applied upon a level turnpike-road, which is required to move 1800 lbs., if applied to drag a canal-boat will move 55,500 lbs., both at the rate of 2-1/2 miles per hour. But we want economy in time as well as economy in the application of motive power. It has been attempted to apply speed to canal travelling. Up to four miles an hour the canal can convey an equal weight more economically than a railroad; but after a certain velocity is exceeded, that is 13-1/2 miles an hour, the horse on the turnpike-road can drag as much as the canal-team. Then comes in the great advantage of the railroad. The same force that is required to draw 1900 lbs. upon a canal, at a rate above 13-1/2 miles an hour, will draw 14,400 lbs. upon a railway, at the rate of 13-1/2 miles an hour. The producers and consumers are thus brought together, not only at the least cost of transit, but at the least expenditure of time. The road, the canal, and the railway have each their distinctive advantages; and it is worthy of note how they work together. From every railway station there must be a road to the adjacent towns and villages, and a better road than was once thought necessary. Horses are required as much as ever, although mails and post-chaises are no longer the glories of the road; and the post finds its way into every hamlet by the united agency of the road and the railway.