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On the Tides of the River Hugli. 
[Nov. 
The ease just considered is the most simple, and of frequent occurrence in the 
backwater creeks of all large deltas ; also in the many minor outlets of large rivers, 
which are only partially sensible to the influence of the upland freshes, and are, in 
some cases, closed entirely from the main river in the dry months. 1 have yet to 
adduce the more complicated case of a river subject to constant upland supply, be- 
sides the daily influence of tides. 
In a running stream it has already been laid down experimentally* and theore- 
tically, that any obstruction or resistance offered to the stream, by river, or other 
contrivance, or by the meeting of an opposite current, will have the effect, by what 
is called afflux, of raising the level at the point where it is applied, and also of affec- 
ting the level for a considerable distance up the stream, to far beyond the additional 
level created by the obstruction at the point itself. 
This principle alone is suffieientto warrant my placing the point -f- in the Hoogly 
tides, (diagram 2), where an inland supply of water always exists, although vary- 
ing in quantity and force, above the level of A A. In proportion as the inland sup- 
ply is withdrawn in the dry season, will the level of -f- approximate to A ; and it is 
by no means certain, that in the dry months of April and May, it does notreach the 
level of A. Of this, however, we have not sufficient data to decide, nor, 1 aui sorry to 
say, do we sufficiently know the limit of the tide at every season, to determine the 
elevation to which -f- rise3 in its extreme, when the freshes are at their maximum 
force. Perhaps some future observations may throw light on this desideratum. 
Meantime, we must rest contented with establishing the variations and relative 
levels at Calcutta, or the n, 6, b, of the 2nd diagram, one step only towards the 
general developement of the subject. 
To assist us to the connection of the two systems of tides, we have an instrumen- 
tal level struck between Calcutta and Diamond Harbour ; and although no corre- 
sponding register of tides has ever been kept at the latter place, to afford us an in- 
termediate chart, still, the lowest water mark has been ascertained within tolerable 
certainty. I have marked its position on the Calcutta chart No. 2, and if this lowest 
water mark correspond, as I believe it must, within some few inches of the lowest 
water mark at Sugar, or answers to the mark — 8ft. Oin. on the chart No. 1, then 
will the dotted line, which I have drawn somewhat more than three feet below the 
base of the Calcutta tide system, represent the level of the Siigar base. 
The differences of mean levels at Sdgar and Calcutta, is thus found to vary be- 
tween four feet, and nine feet, which is the slope between 0 and 0' on diagram 
2. for the two seasons relatively. The 0, at the extreme of the freshes, will be still 
one foot below the highest tide at Sdgar. The O' in January will stand between 
three and four feet, below the high tide at Sdgar, and 1 foot only above the mean 
level or 0 point of the salt lake tides, at the same tide, which is thus ascertained 
to be only two feet above the mean level at Sugar, and consequently, several feet 
at flood tide, below the flood tide at Sugar. 
I must here check myself, before! am led, beyond the patience of your readers, into 
the host of reflexions which appear, as it were, to grow upon the curious facts ad- 
duced, chiefly relating to the peculiarities of our lower delta. 1 hope also, that I 
may not unnecessarily awaken any alarm by publishing the fact, that the high tides 
of the Sand-heads are so much above the level of the lake, in its highest state. 
As the Dutch of Amsterdam look to their Slaaper-dam for their protection against 
annihilation in their periodical inundations, so may we look to the great dis- 
tance over yvhich the tide has to travel, before it could reach the lake and low ground 
in its vicinity, as our safeguard against any like catastrophe. P. T. 
* Vide Du Buat, Part 1, Sec, 3, cl. 3, 
