626 THE FilESH-WATEE LOCHS OF SCOTLAND 
According to Schermerhorn ^ the volume of water in the Great 
Lakes is about 6000 cubic miles, of which Lake Superior contains 
somewhat less than one-half. The mean annual rainfall of the St 
Lawrence basin is about 31 inches, and the mean depth of water 
evaporated from the surfaces of the lakes between 20 and 30 inches.*^ 
The amount of precipitation on the water-surface is therefore little 
more than the amount evaporated from the same area. 
The influence of the Laurentian lakes on the climate of their 
shores is well marked, and was shown as long ago as 1 870 by Alex. 
Winchell.^ The currents on the lakes have been studied by the 
United States Weather Bureau by means of bottles containing a record 
of the locality where they were set adrift, and a request that the 
finder might note the place where they were recovered and transmit 
the record to the chief of the Weather Bureau ; the results of the 
observations are published in their bulletins.* The general courses of 
the currents are also indicated on a chart which Russell reproduces 
on Plate 7 in his Lakes of North America. The difl'erence in the 
currents when the longer axis of the lake coincides with the direction 
of the prevailing winds, and when it lies athwart that direction, is 
very clearly indicated. 
Between the Great Lakes and the estuary the St Lawrence 
widens into the following three lakes : — 
Lake St Francis, 30 miles S.W. of Montreal, 38 miles long by 
about 4 miles broad, with an area of 132 square miles, and an average 
depth of 36 feet. 
Lake St Louis, 9 miles S.W. of Montreal, 15 miles long by about 
5 miles broad, with an area of 75 square miles, and an average 
depth of 30 feet. 
Lake St Peter, 30 miles long by 7 miles broad, with an area of 
200 square miles, and an average depth of 8 feet. The lower end of 
Lake St Peter is 750 miles from the ocean. 
All the great tributaries of the St Lawrence come from the 
north, as well as many small ones which drain the numerous lakes of 
the region. Going from west to east, the St Lawrence receives the 
waters of the following lakes : — 
Lake Nepigon, 30 miles N.W. of Lake Superior, 665 feet above 
sea-level, 70 miles long by 40 miles broad, and with an area of 1450 
square miles, and an average depth of over 540 feet, drains to Lake 
Superior by the Nepigon River. 
1 Op. cit. , p. 282. 
2 Tlios. Russell, " Depth of Evaporation in tlie United States," Mo7ithly 
Weather Report, U.S. Signal Office, Sept. 1888. 
3 " The Isothermals of the Lake Region," Proc. Amer. Assoc., vol. xvi. p. 106, 1870. 
* U.S. Department of Agriculture, Weather Bureau, Bulletin B. 
