Page:The wealth of nations, volume 3.djvu/95

 to the number and weight of the carriages, which are likely to pass over it. The depth and the supply of water for a navigable canal must be proportioned to the number and tonnage of the lighters, which are likely to carry goods upon it; the extent of a harbor to the number of the shipping which are likely to take shelter in it.

It does not seem necessary that the expense of those public works should be defrayed from that public revenue, as it is commonly called, of which the collection and application are in most countries assigned to the executive power. The greater part of such public works may easily be so managed, as to afford a particular revenue sufficient for defraying their own expense, without bringing any burden upon the general revenue of the society.

A highway, a bridge, a navigable canal, for example, may in most cases be both made and maintained by a small toll upon the carriages which make use of them; a harbor, by a moderate port duty upon the tonnage of the shipping which load or unload in it. The coinage, another institution for facilitating commerce, in many countries, not only defrays its own expense, but affords a small revenue or seigniorage to the sovereign. The post-office, another institution for the same purpose, over and above defraying its own expense, affords in almost all countries a very considerable revenue to the sovereign.

When the carriages which pass over a highway or a bridge, and the lighters which sail upon a navigable canal, pay toll in proportion to their weight or their tonnage, they pay for the maintenance of those public works exactly in proportion to the wear and tear which they occasion of them. It seems scarce possible to invent a more equitable way of maintaining such works. This tax or toll, too,