1829.J 
On the Tides of the River Hugli. 
291 
the pouring of the rivers into the hay of Bengal, during the months of August and 
September, and the change of wind at the end of October, give the currents a set in 
the contrary direction, and gradually restore the sea and the river to the state they 
were in, in March. 
The effect of the two monsoons upon the currents, and the height of the sea, in 
the Bay of Bengal, may, therefore, be considered as that of two long unequal tides, 
during the year, eight months of flood, and four months of ebb- 
In conformity with these periodical local causes, partial ones have a correspond- 
ing effect : thus strong southerly winds raise the tides in the Hugli, whilst north- 
erly ones depress them. 
The freshes, or floods of the rivers, are a prominent periodical local cause, ope- 
rating upon the tides of the Hugli at Calcutta. 
The Ganges begins to rise from the melting of the snow, as early as the begin- 
ning of May, but its rising does not sensibly affect the Hugli till the beginning 
of July; at that period, so large an accession of water is thrown into the Hugli, 
that its level is bodily raised both at high and low water. The last is so remark- 
able, that the low water of the freshes (neap tides) is higher than the high water 
(neap tides also) of the dry season, by several feet. 
The Damoda and western small rivers, or mountain streams, contribute very ma- 
terially to the swelling of the Hfigli; and it is, probably, the influence of the Da- 
moda, the Rupnarain, the Tongoracolly, the Hidgelee, and even the Balasore river, 
(the latter situated beyond the mouth of the HOgli,) that occasions the height of 
the low water, by their acting as a dam, and preventing the ebbing of the waters 
from the Ganges, and higher streams, quickly into the sea. 
There is another local affection of the tides, the cause of which I cannot satisfac- 
torily explain. In the North-east monsoon, the night tides are thehighest, whilst in 
the South-west monsoons, the day tides are the highest. 
A conjecture may be hazarded, that as, in both monsoons, the wind is generally 
higher during the day than in the night, that the wind in the South-west monsoon 
raises the day tide, whilst in the North-east monsoon the wind, during the day, 
withholds and depresses the day tide; but that is not entirely satisfactory, in as much 
as the wind cannot possibly be uniform, whereas the fact of the higher tides during 
the day in one monsoon, and during the night in the other, is beyond doubt ; besides, 
the latter is very much more than the former, being as much as two feet, whereas 
the former is seldom more than one foot The night tides in the North-east monsoon 
are also more uniform, in this respect, than the day tides in the South-west monsoon. 
Should it appear from future observation that the wind be the cause, it will prove 
that the depressing effect of the northerly wind, has much more influence upon the 
tides, than the increase by the southerly ones; or it may be, that the absence of the 
wind leaves the tides more freedom to act. I come now to general causes. 
The horizontal parallax of the moon invariably affects the tides ; when that is high, 
the tides are high, and vice versa , to such a degree of correctness, that allowing for 
local causes, I could venture to construct a table for a year in advance, that should 
not vary two inches, from the actual tides. 
When the parallax is highest, on the second, or third day, after the full or change 
of the moon, the highest tide will correspond with these days, as that is the natural 
period of its greatest height : should the parallax be decreasing, the highest tide will 
be on the day of the full, or change; and should the parallax he decreasing, and 
near to its lowest, and increase again after the natural period has passed, the high- 
est tide will he on the fourth day, after the full or change of the moon. 
The difference of effect between the high or low parallax of the moon, upon 
the height of the tides, is about two feet, frequently much more; and as its varia- 
tion, as to the time, is shewn to be four days, this is of importance to all mariners, 
as enabling them, in cases of danger, to ascertain by their nautical ephemcris, the 
true state of the tides. No longer need they trust to the partial observation, and 
equally partial theory founded thereon of pilots and seamen, most of whom have 
a notion that the dark spring tides are always the highest, that the night tides are 
higher than the day tides, and that the highest tide must always occur on the se- 
cond or third day after the full, or change ; whereas the parallax of the moon will 
effectually supercede this uncertainty, and either warn a mariner with his bark on 
a shoal not to wait till the second day, and lose the springs, or save him from de- 
spair, because these days may have passed, and induce him to wait with confidence 
till the fourth day, after the full or change, for the highest tide, as the case may be. 
The parallax of the moon will assuredly indicate the height of the tides all over 
the world ; this general cause, therefore, must be applicable at all places. The fol- 
