National Geographic : 1897 Sep
MODIFICATION OF THE GREAT LAKES If the volume of a lake were invariable, and if its water were in perfect equilibrium 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 temporary 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 corresponding 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 discnst by Wm. T. Blount, in Ann. Rept. Chief of Engineers, U. S . A., for 1894, part 6, pp. 3431-3435.