32 



SCIENCE 



[N. S. Vol. XXVII. No. 67& 



Btreams along belts of weaker rocks." It is not 

 clear why the technical term, baselevel, should 

 be printed as two words in a report which 

 makes a single word of waterfall. 



W. M. D. 



DEFLECTION OF RIVERS BY THE EARTH's 

 ROTATION 



The sufficiency of the earth's rotation to 

 deflect rivers has now been debated many years. 

 The deflective force is well understood to be 

 independent of azimuth and to increase with 

 the sine of the latitude, but to be so weak as 

 to be of questionable value in spite of its 

 persistence. Nevertheless, the observations of 

 von Baer and others regarding Russian rivers, 

 the well-marked asymmetry of the radial val- 

 leys on the great fan of Lannemezan in south- 

 western France, and the occasional instances 

 of unsymmetrical valleys reported in diilerent 

 parts of this country by Kerr, Gilbert and 

 others, have kept the matter in the mind of 

 geographers. As to the Russian rivers, par- 

 ticularly the Volga, where a long-maintained 

 right-handed tendency has resulted in a strong 

 inequality of valley-side slopes, no efficient 

 explanation in place of the deflective force 

 of the earth's rotation has been ofliered. As 

 to the radial valleys of the Lannemezan fan, 

 L. A. Fabre has given good reasons for regard- 

 ing their steeper right and less steep left sides 

 as dependent on the westerly source from 

 which their rains usually come; and as to the 

 greatest of our own rivers, for which the de- 

 tailed maps of the Mississippi river commis- 

 sion give unusually accurate quantitative 

 measures of lateral erosion, the studies of I. 

 Bowman, published in Science a few years 

 ago, leave little doubt that the rotation of the 

 earth, which would turn this south-flowing 

 river westward, has less control than the pre- 

 vailing winds, which brush it eastward. 



A presumed effect of deflection upon river 

 courses has been pointed out by certain Aus- 

 trian geographers, who have noted that some 

 of their larger rivers, especially the Danube, 

 turn to the left in long curves, convex to the 

 right, while passing across alluvial plains be- 

 tween notches in rock ridges ; the left-handed 

 turning of the river curves being the necessary 



result of the right-handed pressure of the river 

 between its pairs of (relatively) fixed points. 

 Had the original course of these rivers been 

 direct, from notch to notch, the deflective force 

 of the earth's rotation, even though quanti- 

 tatively weak, would have been at least quali- 

 tatively appropriate to bring about the exist- 

 ing curved courses. 



deflected rivers in AUSTRALIA 



Few examples of deflected rivers have been 

 noted in the southern hemisphere; hence an 

 especial interest attaches to some instances 

 reported by T. G. Taylor in Australia (" A cor- 

 relation of contour, climate and coal; a con- 

 tribution to the physiography of New South 

 Wales," Proc. Linn. Soc. N. S. W., XXXI., 

 1906, 517-529). It is pointed out that several 

 members of the Murray river system, on the 

 inner plains west of the mountains back of 

 Sydney, exhibit a persistent tendency to turn 

 to the left, while sweeping around long curves 

 convex to the right. It is urged that as the 

 rivers flow for several hundred miles across 

 a nearly level district where there are no rock 

 outcrops to determine their courses, the earth's 

 rotation should become a prime factor in 

 guiding them. 



It is with regret that we have to conclude 

 that the explanation offered by Taylor for 

 these left-curving rivers is not valid. There 

 is no indication that the rivers are held in 

 rock notches at the ends of their curves, and it 

 is, moreover, evident that if the reason given 

 for the Danube curves were applied to the 

 Australian rivers, they ought to turn to the 

 right in long curves convex to the left. The 

 case would then be somewhat analogous to that 

 of cyclonic winds, which being deflected from 

 the barometric gradients by a right-handed 

 force in the northern hemisphere, there curve 

 to the left in spiral inflows to low pressure 

 centers ; or by a left-handed force in the south- 

 ern hemisphere, there curve to the right. It 

 is implied by Taylor that the Murray river 

 branches were originally straight, presumably 

 on lines now indicated by the tangential pro- 

 longation of their upper waters ; hence the 

 greatest amount of deflection should have been 

 in the lower course of each curved branch ; but 



