6 



Prof. J. Thomson on the Origin of 



[May 4, 



A^Bp AjB^, &c. in fig. 2, and wliicli may be designated in general as 

 AB, if the Hne be level, the water-pressure must be increasing from A 

 to B, on account of the centrifugal force of the particles composing that 

 line or bar of water ; or, what comes to the same thing, the water- 

 surface of the river will have a transverse inclination rising from A to 

 B. The water in anj stream-line CD E* at or near the surface, or in 

 any case not close to the bottom, and flowing nearly along the inner bank, 

 will not accelerate itself in entering on the bend, except in consequence 

 of its having a /aZZ of free level in passing along that stream-line t. 



But the layer of water along the bottom, being by friction much re- 

 tarded, has much less centrifugal force in any bar of -its particles ex- 

 tending across the river ; and consequently it will flow sidewise along 

 the bottom towards the inner bank, and will, part of it at least, rise up 

 between the stream-line and the inner bank, and will protect the bank 

 from the rapid scour of that stream-line and of other adjacent parts of 

 the rapidly flowing current ; and as the sand and mud in motion at the 

 bottom are carried in that bottom layer, they will be in some degree 

 brought in to that inner bank, and may have a tendency to be deposited 

 there. 



On the other hand, along the outer bank there will be a general ten- 

 dency to descent of surface-water which will have a high velocity, not 

 having been much impeded by friction ; and this "uill wear away the 



* This, although here conveniently spoken of as a stream-line, is not to be supposed 

 as having really a steady flow. It may be conceived of as an average stream-line in a 

 place where the flow is disturbed with eddies or by the surrounding water commingling 

 with it. 



t It must be here explained that by the free level for any particle is to be understood 

 the level of the atmospheric end of a column, or of any bar, straight or curved, of par- 

 ticles of statical water, having one end situated at the level of the particle, and having 

 at that end the same pressure as the particle has, and having the other end consisting 

 of a level surface of water freely exposed to the atmosphere, or else having otherwise 

 atmospheric pressure there ; or, briefly, we may say that the free level for any particle of 

 water is the level of the atmospheric end of its pressure-cohmn, or of an equivalent 

 i4eal pressure-column. 



