473 



Supposing, therefore, my calculations to be correct, and not 

 taking into account the effects of vegetation, it must have taken 

 14,400 years to raise the head of the delta above high tide to its 

 present level. If to this be added the probable time it took to 

 raise the beds of the ocean above high water, even with such a 

 powerful agent at work as the Irrawaddy, ages on ages must have 

 elapsed since this silting up process began. From the great depths 

 of the river, and its liability to change its course, relics of man, and 

 bones of existing animals, may hereafter be found even more than 

 100 feet below the sea, at different points throughout the delta. 

 Caution should therefore be observed in ascribing antiquity to such 

 relics, nor should they be considered a proof of the subsidence of the 

 land. 



Before commencing this survey, Lieutenant Walker of the Bengal 

 Engineers pointed out to me the inaccuracy of M. Du Buat's rule 

 for calculating the discharge of rivers. I also, while engaged on the 

 survey, discovered that the fundamental rule for finding the mean 

 and bottom velocities, by the known surface velocity, was also inaccu- 

 rate. The rule is, where the surface velocity v is expressed in inches, 

 the bottom velocity equals (^i; — 1)^ 



But even when the river was at its lowest, the bed, which consisted 

 of sand, could only have withstood half the velocity calculated by 

 this rule ; and when the river was in flood, even large boulders would 

 have been swept along by the current. To this fact I beg to draw 

 particular attention, for at no point did I find the bed to consist of 

 such materials as could withstand the calculated velocity ; but the 

 nature of the bed always varied according to the depth and surface 

 velocity. I therefore estimated the bottom velocity by the nature 

 of the bed, instead of abiding by the above rule, and found the 

 mean velocity, by halving the sum of the estimated bottom and known 

 surface velocities. Mr Ellet, however, found on the Mississippi, 

 that where the river was deep, the velocity was always greater at 

 some depth below the surface than at the surface itself. 



Another source of error, — the one of all others most difficult for 

 the engineer to contend against, — is the power rivers have of abrad- 

 ing their beds to considerable depths, and again silting them up to 

 their former level while the flood is subsiding. 



I have had many opportunities of observing this process of scoop- 

 ing out the bed and again silting it up, and I have known it to 



