FETCH, EXPOSURE, AND WAVE-POWER. 87 



this agitation be continued long enough, and sufficient force be 

 employed, the whole of the mud will be lifted from the bottom 

 and be held in suspension. A short time after this little storm 

 has ceased, the mud will have settled to the bottom again, and 

 will be there as if nothing had happened. 



If we place the tub under a tap and let water run into it so 

 that it overflows the sides, and then stir it up, the mud will be 

 carried out of the tub by the overflowing water, and the water 

 in the tub will gradually clear itself. It is therefore evident 

 that, in order to get rid of mud, the agitating power must be 

 accompanied by a transporting one. 



In many bays, currents, if they exist at all, are very feeble. 

 Under such a condition, it is quite possible for mud to exist 

 where the water is comparatively shallow, notwithstanding that 

 at times it may be disturbed by very heavy seas. 



In Algoa Bay (South Africa), fine soft mud exists at the 

 bottom in many places at depths of only 4 or 5 fathoms below 

 low water. 



The waters of this bay which is open to the Indian Ocean, 

 and exposed to heavy storms are frequently much discoloured 

 by this and by line sand. There is no current where this mud 

 lies, excepting an occasional surface-current set up by wind, so 

 the mud is not carried away, but merely subsides when the dis- 

 turbance ceases. 



I have witnessed this on many occasions, and think it well 

 to record the circumstance in order to show that, under the con- 

 ditions named, the mud-test of exposure would be of little value, 

 or might even lead to erroneous conclusions being arrived at. 



In Reid's "Law of Storms," p. 181, I find Captain Webb, 

 describing a typhoon in the China Sea, says, " At daylight we 

 were in 12-fathoms water, with the sea, ivhich was as much mud 

 as water, breaking entirely over us." 



Even where currents exist which are capable of removing 

 mud when agitated, it seems possible that, after the mud has 

 been scoured away from an exposed site through the operation 

 of a great storm, a fresh supply may be brought by rivers or by 

 other agency, and be deposited during the long interval of years 

 which usually elapses between the occurrence of great storms, 

 so that, however useful the mud-test may be in such localities 

 as those referred to by Mr. Stevenson, it is an indication which, 

 in my opinion, ought to be viewed with great caution. It is 



