﻿180 DR. H, C. SORBY ON THE APPLICAlTION OF [May 1908, 



Effects of Current on Sand. 



Pifty-nine years ago, when I was living at Woodbourne, a 

 country-house on the east side of Sheffield, there was at the bottom 

 of the small park a brook entirely under my control. In order to 

 investigate a number of questions, I constructed a place for experi- 

 ment with some self- registering appliances. I could easily regulate 

 and measure the depth and velocity of the current within certain 

 limits. By these experiments, and by observations made in a clear 

 brook at Fulwood (near Sheffield), I came to the conclusion that, 

 when the velocity of the current is about 6 inches per second, sand 

 with grains about a hundredth of an inch in diameter is drifted 

 along slowly, and a surface is produced, grained in the line of the 

 current, but no ripple-marks are formed. When the velocity is 

 somewhat greater than 6 inches per second, ripples are produced. 

 When it is about 1 foot per second, these are well developed and 

 advance about 3 inches per minute, by the sand being washed 

 up on the exposed side and deposited on the other ; which velocity 

 may be looked upon provisionally as an average for undoubted 

 drifted ripples. If the velocity attains 18 inches per second, the 

 ripples are destroyed by the washing-away of the sand ; but the 

 surface may still show graining in the line of the current. Much 

 depends, however, on whether sand is or is not being deposited from 

 above; since, when it is, ripples are produced at a somewhat 

 lower velocity and advance more quickly. These results applied to 

 the case of water varying from 1 to 8 inches in depth, and might 

 be very different in the case of much deeper water. I have long 

 felt that such experiments ought to be conducted on a much larger 

 scale, but have never had the opportunity in a suitable and con- 

 venient place, free from .disturbance. In the present state of the 

 subject it may be assumed that, in the case of moderately-fine 

 sand, the well-developed ripple-drift, so common in certain rocks, 

 indicates a current with a mean velocity of about 1 foot per 

 second. 



Assuming that the sines of the angles of rest are a fair measure 

 of the friction which must be overcome to move the sand when at 

 rest, and to continue the motion when drifting over the same sort 

 of sand, and also that the effective action of a current of about 

 1 foot per second varies as the velocity, I calculate out the following 

 table. It must, however, be looked upon as little more than a 

 provisional illustration, since it is possible that many other factors 

 should be taken into account. They are not velocities, but the 

 relative forces needed to more the sand along the bottom, where 

 the current would be much reduced by friction. 





Table II. 









Coarse sand. 



Fine sand. 



Very fine. 



Angles of rest 



41° 49* 



24' 26° 



30° 33° 



Sines of angles 



-65 -75 



•56 -59 



•50 -54 



