"معاملات الجمعية الجغرافية في بومباي. من يناير ١٨٦٣ حتى ديسمبر ١٨٦٤. (تحرير السكرتير.) المجلد XVII." [‎٨‎٨] (٥٢٤/١١٧)

Ixxxviii proceedings of the
In bis first letter, dated 19th March, Captain Wherland gives it a8
his opinion, that I “ under-estimate the effect of the wind on the time of
high-water,” and says he has experienced a difference of “ more than
twenty minutes on several occasions
To this I would reply that, severe storms blowing for any considerable
length of time, may perhaps influence the tides to an extent which we
have not data for estimating. But severe storms are exceptional
phenomena, and what we have to do with in estimating the effects of
the winds on the times of tide, is not exceptional cases that may occur
only once or twice (if so often) in the course of twelve months, but the
effect of such winds as are of frequent occurrence, and consequently
differing within moderate limits only from their average force. Now
I think we have no sufficient data to show that their effects are
greater then I have allowed.
In the case of ports like Bombay, within the tropics, where the
prevailing direction of the wind varies with the sun’s declination,
their average effects will be masked under those dependent on the
declination. Hence proper formula, founded on sufficient observations,
would, in such cases, include the mean effects of this disturbing cause,
and it would only be in extreme cases that they could farther materially
alter either the time or the height of the tide.
I am inclined to think the difference Captain Wherland supposes he
has observed, may be explained in one of the following ways :—Either,
depending on the accuracy of the Nautical iUmanac method, he proba
bly computed the time of high-water by it, and finding it did not agree
with observation, he attributed the error to the wind, when in reality
it was owing to the error of the method of computation ; or, perhaps
he counted the time of flood tide as coinciding with the time of slack
water, which has often been done, and whilst the tide really “ made its
mark” at the computed time, the current turned c< more than twenty
minutes” later. Observations of such a kind, to be trustworthy, must
be made at ports for which really good tide tables are prepared; such
as London, Plymouth, Bristol, Liverpool, Brest, &c. Comparisons with
times computed hy the Nautical Almanac method in use, are only
trustworthy at fall and change : immediately after the ‘ quarters ’ the
computed times are erroneous—but much less so between places where
the tides are of about the same age, than between those for which the