THE LATERAL EROSION OF STREAMS 437 



greatest in streams with straight courses — that the force is opposed 

 to the development of meanders and in high latitudes may fully 

 account for the conditions described above. 



The Deflecting Force of the Earth's Rotation 



For a brief elementary review of the principles which give rise to 

 this deflecting force, we may imagine the earth perfectly smooth 

 and friction eliminated. This force has two modes of origin, one 

 being active when the relative motion is in an east-and-west direction, 

 the other when the relative motion is north and south. In case the 

 relative motion is not in any true direction, both modes are active 

 at once, and the deflecting force has two components arising in differ- 

 ent ways, but acting similarly. 



To get these components distinctly in mind we may consider them 

 separately. (For convenience the following abbreviations may be 

 used: F^= force of gravity, F^ = centrifugal force, F^ = deflective 

 force of the earth's rotation.) 



Component of Deflecting Force Arising from East-and-West 

 Relative Motion 



An object upon the earth's surface and at rest with respect to it 

 has a certain gyratory velocity equal to that of the earth's surface 

 at the same latitude. This gyratory motion gives rise to a centrifugal 

 force, acting in a plane normal to the axis of rotation. Owing to 

 the nature of the elhpticity of the earth, a gradient is furnished at 

 every point whereby a part of the force of gravity is caused to oppose 

 and exactly counterbalance this centrifugal force, and the object 

 would have no tendency to move either toward or away from the 

 equator. 



However, if the object had an easterly motion with respect to 

 the earth's surface, it is readily seen that the centrifugal force would 

 be increased and the ellipticity of the earth's surface would no longer 

 be sufficient to cause its counteraction by gravity. The resultant 

 force would furnish a component acting along the earth's surface 

 toward the equator to the right in the northern hemisphere and to 

 the left in the southern. Again, if the object had a westerly motion 

 relative to the earth's surface, the centrifugal force would be decreased 



