THE SUFFICIENCY OF TERRESTRIAL ROTATION FOR THE DEFLECTION 



OF STREAMS. 



By G. K. Gilbeut. 



EEAD APRIL 15, 1884. 



It was long ago perceived that rivers of the northern heniisjihere flowing to the north or to 

 the south should by the rotation of the earth be thrown severally against their east or west banks. 

 It is even many years since it was shown by Ferrel that these tendencies are but illustrations of 

 a more general law, that all streams in the northern hemisphere are by terrestrial rotation pressed 

 against their right banks, and all in the southern are pressed against their left bauks, the degree 

 of pressure being independent of the direction of flow. Yet the question of the sufBciency of the 

 cause for the production of observable modifications in the topography of stream valleys is still 

 an open one. A number of geologists have observed peculiarities of stream valleys which they 

 referred to the operation of the law, while others, including myself, have looked in vain for phe- 

 uominal evidence of its efBciency. Nevertheless it is my present purpose to maintain the sufBciency 

 of the cause. 



So far as I am aware, all those who have attempted to consider analytically the mode in which 

 the lateral tendency arising from rotation should modify the channel or valley of a stream have 

 reached the conclusion that no appreciable results can be produced, and for the most part their 

 conclusions legitimately follow their premises.* My own different conclusion is based upon an 

 essentially difierent analysis of the processes involved. In the celebrated discussion of the sub- 

 ject in the French Academy of Science, it was computed l)y Bertrand that a river flowing in 45° 

 north latitude with a velocity of three meters per second exerts a pressure on its right bank of 

 63^39 of its weight, and he regarded this pressure as too small for consideration.! It has been 

 pointed out by Henry Eutf that the deflecting force, by comliining with gravitation, gives the 

 stream's surface a slight inclination toward the left bank, thereby increasing the depth of water 

 near the right bank, and consequently increasing the velocity of the current at the right. To this 

 increment of velocity he ascribed a certain erosive efiect, but regarded it as less than that assign- 

 able to wind-waves on the same water surface. He therefore accorded a more important influence 

 to the prevailing winds than to the rotation of the earth.J It has been held bj' others that the 

 combination of the deflective force with gravitation is equivalent simply to a slight modification — 

 so i'ar as the stream is concerned — of the direction of gravitation ; and that, the flood-plain of the 

 stream having been adjusted normal to this modified direction of gravitational attraction, no other 

 geological effects are produced. The last was my own view until I perceived the importance of 

 certain considerations, to which I now proceed. 



The foi'm of cross-section of a stream flowing in a straight channel depends on the loading and 

 unloading of detritus, and is essentially stable. It is evident that the form of the cross section 



* 1 hare receatly become cognizant of a discussion by Baines to which this sentence does not apply. See note 

 to the following page. 



t Comptes rendus, XLIX, 1859, p. 658. 



} Annalcn der Chemie u. Pharii acie, IV Siipp. I Band. Leipzig and Ileidellierg, 1865-1806. 



