The portion of these cribs above water is found not to last more than ten or fifteen years; so that it is now recommended to replace them with piers of stone masonry, wherever the material is easy of access. As to the cause of the shore-currents which produce this mischief, Col. Graham says, in one of his Reports,-- "The great power which operates to produce the littoral or shore currents of the lake is the prevailing winds; just as the great ocean current called the Gulf Stream is produced by the trade-winds. The first-mentioned phenomenon is but a miniature demonstration of the same principle which is more boldly shown in the other. The wind, acting in its most prevalent lakeward direction, combined with this littoral current, produces the great power which is constantly forming sand-bars and shoals at all the harbor-entrances on our extensive lake-coasts. To counteract the effect of this great power, upon a given point, is what we have chiefly to contend for in planning the harbor-piers for all the lake-ports intended to be improved. The point which an engineer first aims at, in undertaking to plan any of these harbor-works, is to ascertain as nearly as possible the direction and force of the prevailing winds." The length of the Chicago piers is as follows:--North pier, 3900 feet long, 24 feet wide; south pier, 1800 feet long, 24 feet wide; and they are placed 200 feet apart. Harbors of this kind have been constructed at Chicago, Waukegan, Kenosha, Racine, Milwaukee, Sheboygan, Manitoowoc, Michigan City, and St. Joseph, on Lake Michigan; at Clinton River, on Lake St. Clair; at Monroe, Sandusky, Huron, Vermilion, Black River, Cleveland, Grand River, Ashtabula, Conneaut, Erie, Dunkirk, and Buffalo, on Lake Erie; at Oak Orchard, Genesee River, Sodus Bay, Oswego, and Ogdensburg, on Lake Ontario. For Lakes Huron and Superior it is believed that no appropriations have been made, the scanty population of their shores not seeming as yet to demand it, and those two lakes having in their numerous groups of islands more natural shelter for vessels than Michigan or Erie. Besides these river-harbors, Col. Graham recommends to Government the construction at certain points on the lakes of sheltered roadsteads, or harbors of refuge, to which vessels may run for shelter in bad weather, when it may be difficult or dangerous to enter the river-mouths. These are proposed to be made by building breakwaters of crib-work, loaded with stone, and extending along the shore in a sufficient depth of water to admit vessels riding easily at anchor under their lee. Many lives and much property would undoubtedly be saved every year by such constructions; for it is a difficult matter for a vessel to enter these narrow rivers in a heavy gale of wind, and if she misses the entrance, she is very likely to go ashore. Another very important work to the navigation of the lakes is the deepening of the channel in Lake St. Clair.