The presence of this great body of water modifies the range of the thermometer, lessening the intensity of the cold in winter and of the heat in summer, and gives a temperature more uniform on the Lake coasts than is found in a corresponding latitude on the Mississippi.
The difference between the temperature of the air and that of the Lakes gives rise to a variety of optical illusions, known as mirage. Mountains are seen with inverted cones; headlands project from the shore where none exist; islands clothed with verdure, or girt with cliffs, rise up from the bosom of the lake, remain awhile, and disappear. Hardly a day passes, during the summer, without a more or less striking exhibition of this kind. The same phenomena of rapidly varying refraction may often be witnessed at sunset, when the sun, sinking into the lake, undergoes a most striking series of changes. At one moment it is drawn out into a pear-like shape; the next it takes an elliptical form; and just as it disappears, the upper part of its disk becomes elongated into a ribbon of light, which seems to float for a moment upon the surface of the water.
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.
A large number of facts have been collected by Messrs. Foster and Whitney on the subject of these oscillations of the Lake level; and, in fact, these phenomena have been for a long time familiar to the residents on the Lake shores. They are generally attributed by scientific men to atmospheric disturbances, which, by increasing or diminishing the atmospheric pressure, produce a corresponding rise or fall in the water-level. These are the sudden and irregular fluctuations.
The gradual fluctuations are probably caused by the variable amount of rain which falls in the vast area of country drained by the Lakes. Thus, at Fort Brady, where the mean of five years' observations is 29.68 inches, the extremes are 36.92 and 22.44.
An idea has been long prevalent among the old residents, derived from the Indians, that there is a variation of the Lake surface which extends over a period of fourteen years,—that is, the Lakes rise for seven years, and fall for seven years. The records kept by accurate observers at various points on the Lakes for the last ten years do not seem to confirm this theory; but it has been well established by the recent observations of Colonel Graham, at both ends of Lake Michigan, that there is a semi-diurnal lunar tide on that lake of at least one third of a foot.
The evaporation from this great water-surface must be immense. It has been estimated at 11,800,000,000,000 cubic feet per annum; and in this way alone can we account for the difference between the volume of water which enters the Lakes and that which leaves them at the Falls of Niagara. Immense as is the quantity of water which pours over the Falls, it is small in comparison with the floods which combine to make up the Upper Lakes.