320 
On the Tides of the River Hugli. 
[Nov. 
All who have devoted any attention to the subject will be aware of the absolute 
necessity, in order to draw any general conclusions from such varying phenomena, 
of reducing the observations to mean results, I have essayed to make this reduc- 
tion both with the S&gur and Calcutta tides, as will be seen in the annexed charts. 
The method of making this reduction may, perhaps, be matter of opinion. My 
first idea- was to deduce the mean level from the average of the spring extremes, 
flood and ebb, compared with the average of the neap extremes, flood and ebb; thus 
striking an average from the tides of the full and new moon, which are necessarily 
not of the same extent. On comparing a result from such data, however, with the 
result which is obtained from the general mean of the whole monthly floods and 
ebbs, there appears to be too sensible a difference in the position of the curve of 
mean level obtained from the two methods ; and I therefore incline to give prefer- 
ence to the general mean, which is that described on the annexed charts. At S£gur 
either method will give a satisfactory result, the difference being scarcely sensible. 
With the S£gur tides, the first essential consideration is to fix as base of the system, 
what may be called the universal mean level of the whole year, or perhaps what is 
the same thing, (where the position of the observer is so near the mouth of the river,) 
the general mean level of the tides of the bay of Bengal; and this I have supposed 
not to he very different from what I have assumed as the zero or base-line of the 
S&gur chart No. 1.* Wc then become sensible, on viewing the phenomena laid off 
on this base, what is the real and true influence to be ascribed to the prevailing 
winds of the two monsoons, and what the effect of the discharge of upland water 
during the freshes. The path of mean level is our certain guide to this. 
In January we find the monthly mean level 1ft. lin. below zero. In June the curve 
reaches 1ft. 7in. above zero. The level appears again to sink with the subsiding of 
the southerly winds to August ; and what is at first view singular, docs not again rise 
with the greatest discharge of the freshes in September to the height attained by the 
influence of the southerly winds of May. The influence of the freshes in 18558-29, 
was evidently at its maximum before the Equinox ; but I fancy further observations 
would show this maximum to vary, as at Calcutta, between August and September ; 
depending partly upon several circumstances, such as whether the first springs of 
September happen with a full or new moon. 
The view of the Calcutta chart No. 2, is somewhat more varied. The path of 
mean level, which in the former chart No. 1, performs a kind of double curve, from 
the effects of the opposite monsoons, here presents one single undulation of much 
greater extent, bespeaking the preponderating agent upon its levels, to be the dry or 
full state of the upland rivers. The lowest state of the river appears to happen in 
January, in the height of the northerly monsoon. The curve then rises with the ap- 
proach of the southerly mousoou till it has reached -f- 1ft. 8in. in May. The rise is 
then progressive to its extreme elevation in August or September at -f- 6ft. 9in. From 
this amount -J- 6ft. 9in. however, some little must be deducted for the depression 
created by the north winds of January; the total influence of the freshes cannot 
therefore he said to amount to the whole of the above quantity. 
For the perfect connection of the two charts, it cannot but be regretted that we 
have uot more satisfactory data from instrumental admeasurement, by which to 
assign the exact position of the base line of the one system of tides, upon the chart 
of the other system. Speculation might then give place to certainty ; and the levels 
of Calcutta and its vicinity, relatively with the tides of the Sand Heads, would be 
known beyond doubt. 
On the chart. No. 2, I have traced the path of mean level, which obtains in the 
eastern canal, which is connected with the series of salt lakes or marshes fed by 
the tides of the Bay, and which forms the eastern boundary to the town and sub- 
urbs. The extreme variation of the tide, which obtains in this canal throughout the 
year, is only four feet, and that due to season, or variation of mean level, only 2ft 
4in. Now as this system of lakes has no communication with the river of any 
moment, (except by Tolley’s nulla,) and communicates with a series of tide-creeks 
fed directly from the Bay, whose mean level is affected by inland drainage even 
less than this quantity 2ft. 4iu., it seems natural to turn to this as the link to guide 
us to the solution of the difficulty of connecting the Calcutta andSdgur tide-systems. 
* ^ ie c ^ art assumed the mean between the levels of April, May, Janu- 
ary and February. I believe, however, it would be more correct to ascribe a greater 
influence to the southerly winds, than to the northerly ; and that the proper base 
should be six inches, or more, below that assumed. 
