86 SEVENTH REPORT—1837. 
The insoluble matter held in suspension by the columns of flood and 
ebb amounts to 29 cubic inches for each cubic yard of water on the 
flood, and 33 cubic inches on the ebb,—the matter on the ebb ex- 
ceeding that on the flood; so that 48,065 cubic yards of silt are de- 
tained by the banks outside the Rock Narrows each tide, except that 
part which the succeeding ebb disturbs. 
The excess of silt thus accumulated from the 730 refluxes of a year’s 
tides, amounts to 35,087,450 cubic yards, equivalent to a layer of 
mud, if equally disseminated, 21 inches thick over the first tidal area ; 
one-third, however, of this is disturbed and carried over the second 
tide area; and, further, the deposit is lessened by the natural shrink- 
ing as it is consolidated (namely, into half its original bulk). The 
annual tangible deposit is therefore 11,695,817 cubic yards, which, 
equally disseminated, would produce a uniform increase-of the banks, 
and decrease of water, in the Channel, of 7 inches. The subsidence, 
however, is not in equal proportions, but is directed towards certain 
knolls, margins of banks, and: elongated spits, which protrusions cause 
scouring and choakings of particular channel-beds. Captain Denham 
further pointed out instances of the rapid filling up of certain channels, 
as well as the changes due to sudden change of circumstances, and the 
importance of preserving the present conditions of the harbour, inas- 
much as regards the continuance of the present back-water in its full 
amount. The cross-set of the Irish Channel limits the extension 
outwards of the shoals; but notwithstanding this fortunate natural 
safety-valve, any neglect of those regulations which tend to check 
encroachments on the Channel, would so far facilitate the natural 
tendency to fill up, that in a very short period the part might, be 
closed. Captain Denham pointed out the advantages of the new 
channel as compared with the old. That channel was discovered by 
himself, and is in itself an illustration of the resulting effects of the 
modification of the contour of the general tidal channel, and of the 
benefit to: be derived from closely watching the operations, whether 
natural or artificial, by which it may in any way be altered. 
In 1836, the new channel admitted 8208 vessels, 4077 of which 
could not without it have got in or out under four hours’ delay upon 
each tide. In April 1837, 1571 vessels passed, 760 of which could 
not have passed any other way. The post-office packets have passed 
directly to and fro, with thirty-five exceptions, which exceptions, in 
1832, amounted to 261, involving the necessity of transferring the 
mail and passengers by tender or boat. And, further, out of 29000 
vessels that entered the port in the last two years, only nineteen had 
experienced serious difficulties. 
Having thus shown the surplus quantity of mud which is at each 
tide deposited and added to the banks,—the natural tendency result- 
ing from this accumulation to fill up the channel,—the fortunate ex- 
istence, under the influence of the present conditions of the tidal or 
water-way, of a good and available channel, and the paramount neces- 
sity of securing the existence of that channel by allowing no such 
alteration in the boundaries of the water-way as should, by lessening 
