238 MODIFICATION OF THE GREAT LAKES 



If the volume of a lake were invariable, and if its water were 

 in perfect eqviilibrium under gravity, its surface would be con- 

 stant and level, and any variation due to changes in the height 

 of the land could be directly determined by observations on the 

 position of the water surface with reference to the land ; but 

 these conditions are never realized in the case of the Great Lakes, 

 where the volume continually changes and the water is always 

 in motion. The investigator therefore has to arrange his meas- 

 urements so as to eliminate the effect of such changes. 



Consider first the influence of wind. The friction of the wind 

 on the water produces waves. These are temporar}^ and practi- 

 cally cease in periods of calm ; the perpetual ground-swell of 

 the ocean is not known on the lakes. The friction of the wind 

 on the water also drives the water forward, producing currents. 

 The water thus driven against the lee shores returns in under- 

 currents, but the internal friction of the water resists and delays 

 the return, and there is consequently a heaping of the water 

 against lee shores and a corj-esponding lowering of its level on 

 other shores. During great storms these differences amount to 

 several feet, reaching a maximum in Lake Erie ; in October, 

 1886, a westerly gale is reported to have raised the water 8 feet 

 at Buffalo and deprest it 8 feet at Toledo.* For light winds the 

 changes of level are much smaller, but they are nevertheless 

 appreciable, and they have even been detected in the case of the 

 gentle " land and sea " breezes which in calm weather are created 

 by the diurnal cycle of temperature change on the land. 



The water is also sensitive to atmospheric pressure. If the 

 air prest equally on all parts of the lake surface the equilibrium 

 of the water would not be disturbed ; but its pressure is never 

 uniform. As shown by the isobars on the daily weather map, 

 there are notable differences of pressure from point to point, and 

 within the length of one of the Great Lakes these often amount 

 to several tenths of a barometric inch. A column of mercury 

 0.1 inch high weighs as much as a column of water 1.3 inches 

 high ; and whenever the atmospheric pressure at one point on a 

 lake exceeds the pressure at another point by the tenth of a baro- 

 metric inch, the water level at the first point is, in consequence, 

 1.3 inches lower than the water level at the second point. When 

 a cumulus cloud forms over the water there is a reaction on the 



* Science, vol. VIII, pp. 34, 391. The effect of a storm in October, 1893, is ably disciist 

 by Wm. T. Blount, in Ann. Rept. Chief of Engineers, U. S. A., for 1894, part 6, pp. 

 3431-3435. 



