14 
On the Tides of the Rive r Hugli. 
rjAN. 
your every day readers, who may not have considered hydraulic subjects with 
ireat attention, 5 after some additional illustration. The subject itself would indeed 
readily bear dilating into a much greater compass than would probably suit the 
nature of your valuable little periodical ; perhaps, also, infinitely beyond the patience 
of your readers: and my fear of trespassing on the bounds of either may very 
easily have wrapped in enigma what a very little explanation cannot fail to render 
perfectly intelligible. , , . 
The notice, in your tenth number, of Mr. Kyd’s labours, rendered it unnecessary 
to recite, the peculiarities of the tide, and I, therefore, confined myself to the reduc- 
tion of his observations to mean results, and the comparison of these mean results 
with other tides at the mouth of the river, and in the Suit Lake immediately in our 
neighbourhood. ... 
In drawing this comparison, it was impossible to avoid recognising the two prin- 
ciples which I endeavored to illustrate by the two marginal diagrams inserted in 
the paper ; first, that in a tide channel or creek through wh ch there is no discharge 
or drainage of upland water, and where the elongation of its course is sufficient to 
dissipate the effects of the diurnal tide at the mouth of that creek, the surface of 
the water at this place, where the influence of the daily tide is not perceptible, (the 
point 0 of the diagram) will be below the level of the highest flood tide at the 
mouth of the creek. Secondly, that in the mouths of rivers, or in tideways, 
through which there is any constant and considerable discharge of upland water 
into the sea, the surface of the water at the limit to which the influence of the 
daily tide extends up that river, (the point 0 of the 2nd diagram) must be above the 
flood tide-level at the mouth. 
This latter principle has been already very generally noticed, although, without 
any minute elucidation of the former. In my own limited researches I can find no 
recognition in any works upon the subject, but from some casual notices upon the 
water levels at New Orleans, a place situated precisely in the same manner as Cal- 
cutta, within the tide-way of the Mississipi, and with the salt or marine lake Pout- 
chartrain immediately behind it, the river under its walls, being subject to an annual 
fresh or swelling of its waters, while the lake behind exhibits a nearly even and con- 
stant level, with still a small daily fluctuation from the gulf tides, witli which it 
communicates directly. Also from the elaborate accounts of the tides in Holland, 
reduced to the Amsterdam standard given in Lalande, I almost conceive it impos- 
sible that the principle could have been overlooked. And some careful enquirer 
among the works of the Dutch engineers, may, very likely, discover much more 
curious and even better detailed particulars than I have been able to collect. 
The favorite cases which are most generally adduced in theoretical treatises on 
tides, have all some opportunity of the application of lunar attraction. Such as the 
Mediterranean, Baltic, German, and Northern seas, which from their extent are 
of the surface of the sea at the mouth. 
•=> beyond the diurnal rise and fall 
It matters not, as described of the Amazon 
and other rivers and estuaries, whether more than one tide interval elapses between 
the mouth and the point 0 \v;here the effect of the tide is finally dissipated. The 
general principle winch I have in my former diagram 1 . (here repeated for f-,r 
lity of reference,) endeavoured to represent, will not be vitiated by the circumstaiw 
It may be better understood, perhaps, by the following illustration H 
appose the A and B of the preceding di air ram to be renl-iemt i._ , . 
the part above, from A O B, towards 0, to represent a cistern t\ c . ocks » al 
SO constituted, .La" 
