Thunder-storms of great violence are not unusual, and sudden gusts of wind spring up on the Lakes, and those who navigate them pass sometimes instantaneously from a current of air blowing briskly in one direction into one blowing with equal force from the opposite quarter. The lower sails of a vessel are sometimes becalmed, while a smart breeze fills the upper. The storms which agitate the Lakes, though less violent than the typhoons of the Indian Ocean or the hurricanes of the Atlantic, are still very dangerous to mariners; and, owing to the want of sea-room, and the scarcity of good harbors, shipwrecks are but too common, and frequently attended with much loss of life. A short, ugly sea gets up very quickly after the wind begins to blow hard, and subsides with equal celerity when the wind goes down. The fluctuations in the level of the waters of these lakes have attracted much attention among scientific observers; and as early as 1670, Father Dablon, in his "Relations," says,--"As to the tides, it is difficult to lay down any correct rule. At one time we have found the motion of the waters to be regular, and at others extremely fluctuating. We have noticed, however, that at full moon and new moon the tides change once a day for eight or ten days, while during the remainder of the time there is hardly any change perceptible.... Three things are remarkable: 1st. That the currents set almost constantly in one direction, namely, towards the Lake of the Illinois, [Michigan,] which does not prevent their ordinary rise and fall; 2d. That they almost invariably set _against_ the wind,--sometimes with as much force as the tides at Quebec,--and we have seen ice moving against the wind as fast as boats under full sail; 3d. That among these currents we have discovered the emission of a quantity of water which seems to spring up from the bottom." Father Dablon is of opinion that the waters of Lake Superior enter into the Straits by a subterranean passage. This theory, he says, is necessary to explain two things, namely: 1st. Without such a passage, it is impossible to say what becomes of the waters of Lake Superior. This vast lake has but one visible outlet, namely, the River of St. Mary; while it receives the waters of a large number of rivers, some of which are of greater dimensions than the St. Mary. What, then, becomes of the surplus water? 2d. The difficulty of explaining whence come the waters of Huron and Michigan. Very few rivers flow into these lakes, and their volume of water is such as to fortify the belief that it must be supplied through the subterranean river entering the Straits.