206 Transactions. 



Art. XX. — The Influence of the EartKs Rotation on the Course 

 of the Rivers on the Canterbury Plains. 



Bv F. W. HiLGENDORF, M.A., D.Sc, Lincoln College. 



[Par< of an Address delivered to the Canterbury Philosophical Institute, 

 \st November, 190.').] 



Plate X. 



William Ferrel, in 1859, published a paper in the Mathematical 

 Monthly announcing the law now known as " Ferrel' s law": 

 *' If a body move in any direction on the earth's surface, there 

 is a deflecting force arising from the earth's rotation which de- 

 flects it to the right in the Northern Hemisphere and to the 

 left in the Southern." The application of this law to the ex- 

 planation of the course of the trade-winds is familiar to every- 

 body, and it must also apply to streams. There are, of course, 

 many other factors which come into play to decide the course 

 of the stream, and the question is one that has aroused consider- 

 able discussion. 



G. K. Gilbert, writing on " The Sufficiency of Terrestrial 

 Rotation for the Deflection of Streams," in the " American 

 Journal of Science," vol. xxvii (1884). says he started by con- 

 sidering the rotation of the earth quite unnoticeable, owing to 

 greater differences due to hardness of rocks, slope, &c., but 

 nevertheless, on giving the matter full consideration, is now 

 compelled to write supporting the contention that the terrestrial 

 rotation is sufficient to cause deflection. In a discussion in the 

 French Academy of Science, Bertrand demonstrated that a river 

 flomng in S. lat. 45°, with a velocity 3 metres per second, would 

 exert on its left bank « 33^39- of its weight, and he regarded this 

 as too small for consideration. Henry Buff pointed out that 

 the influence of rotation combined with that of gravity would 

 be to heap the waters up on the left side, make them a litt'e 

 deeper there, and so mcrease the velocity shghtly, so that the 

 terrestrial rotation increases the transporting, and therefore cor- 

 rading, power on the left bank. It is true, however, that he 

 regarded this as of less effect than the wind- waves on the same 

 surface. It has been held by others that the influence of the 

 rotation merely amounts to a slight change in the direction of 

 gravitation — that a river flowing down an incline to the sea 

 will flow not straight do^v^l, but slightly to the left, and that 

 that is all. The river will take up this course, adjust itself to it, 

 and then nothing further will happen. Gilbert himself held this 

 opinion^^^for some time, but saw fit to abandon it. 



