1852.) OF THE ROYAL INSTITUTION. 155 
divides the waters of the Mexican gulf from those of Hudson’s 
Bay ; — and then, bifurcating, one fork proceeds on the north side 
of Lake Superior eastwards towards Labrador, in groups of broken 
hills, while the other fork passes south-east as a rough and high 
country into the lowlands of the United States. It therefore occupies 
an oblong crescent-shaped hollow, with a general direction rather 
to the north of east. It has literally thousands of lakes on its 
north, and hundreds on its immediate south. It is 1750 miles 
round, 420 miles long, and 163 in extreme breadth. It is 597 
feet above the Atlantic. Its greatest known depth is 792 feet. 
Soundings of 300, 400, 600 feet are common ; but extensive shallows 
and flats prevail in parts. 
The hydrographic basin of Lake Superior is singularly small, parti- 
cularly on the south shore, where the tributaries of the River Mis- 
sissipi and Lake Michigan often approach within 5 and 10 miles of 
the lake. It seems to be its own fountain head. 
The water is clear, greenish, extremely pure, pleasant to the 
taste, and soft from the nearly total absence of limestone from these 
regions. An imperial pint only contains <5, part of a grain of 
mineral matters -— carbonates of lime and magnesia, sulphate of 
lime, peroxide of iron, and the oxide of manganese. 
The average annual temperature of the water is 40° F.; being 
about the same as that of the ocean at certain great depths. In 
June, the lake is often covered with ice; and in the middle of 
July, the surface-water freezes in the morning — with patches of 
snow in the clefts of the rocks. At this period of the year, or a few 
days later, the smaller lakes on the north are steadily at 72° and 
14°. F. 
Lake Superior is not undergoing secular drainage. It is lowest 
in April, and highest by a few feet, in September. The great 
annual variations of rain of these countries produce corresponding 
changes of level. There are no tides, and no cycle of years for lake- 
levels. 
Barometric changes produce curious local oscillations of level. 
Thus the furious rapids, called the Falls of St. Mary, on the river 
of discharge so named, are sometimes left dry. Messrs. Foster and 
Whitney have seen the oscillation come from the centre of the lake 
in a wave 20 feet high — curling over like an immense surge, crested 
with foam, and breaking on the shore, diminishing as it approached 
it. On this occasion (Aug. 1845) it was the harbinger of a violent 
storm.* 
The amount of water leaving the lake is small; for its outlet is 
often shallow, and the current weak. 
asec oath nl tee Rt es eR ee a Se 
* A violent gale of wind, concurring with a local rise of level, will sometimes 
throw large stones or logs of wood 150-200 yards inland, and 30-40 feet above 
the usual water margin—as in three instances seen by Prof. Agassiz (L. Superior, 
pp. 95 and 106), and by Dr, Bigsby. (Journ. Roy. Instn. xviii. 15.) 
