REPORT ON THE TIDES. 
191 
necessity of renewing from time to time these determinations. 
This method of adding a constant quantity was somewhat im¬ 
proved by Mr. Phillips in 1668, who gave, in the Philosophical 
Transactions for that year, a Table showing the variations in the 
interval between the time of the moon’s southing and the time of 
high water. Shortly afterwards, Flamsteed having frequent occa¬ 
sion to pass between London and Greenwich by water, and having 
caused above 80 high waters at Tower Wharf and Greenwich 
to be observed, found that the greatest and least differences 
betwixt the moon’s true southing and the high waters were not, 
as Mr. Phillips had placed them, at the full or new and quarter 
moons, but the greatest nearer to the neaps, and the least to 
the highest spring tides. Previously the Tide Tables had only 
shown the time of that high water which next follows the moon’s 
southing; Flamsteed introduced both in his Tide Table. 
In the earliest years of the Royal Society, some attempts were 
made to set on foot observations of the Tides by some of the 
active members ; but these phenomena have long ceased until 
lately to excite any attention wdiatever in this country. Since 
the establishment of the various Docks at London, the times 
and height of high water have indeed generally been registered 
there in books kept for the purpose; and to these, which scarcely 
deserve the name of observations, must we have recourse if we 
wish to determine the various constants of the expressions which 
furnish the means of calculating the time and height of high 
water, or of ascertaining the agreement betw r een theory and the 
tides in our river. Nevertheless by taking the mean of an im¬ 
mense number of observations, the error is almost eliminated ; 
and in the unfortunate alternative of being obliged to relinquish 
the question altogether, or of making use of these imperfect 
data, I have preferred the latter, and, with the assistance of 
Mr. Dessiou of the Admiralty, have discussed a great number 
of observations made at the London Docks. I have found that 
when the effects of changes in parallax and declination are 
neglected, the agreement between the results of observation and 
the theory of Bernoulli (which so far coincides with that of La¬ 
place,) is very remarkable. 
With respect to the effects due to changes in the moon’s 
parallax and declination, I am not yet able to speak so positively, 
although it is certain that the height of high water is about a 
foot more, at the London Docks, when the moon is nearest to 
the earth than when she is furthest, of course cceteris paribus. 
When the moon is in the equator, the time of high water is re¬ 
tarded about half an hour from what obtains when she is in her 
greatest declination; the height is also about six inches less. 
