MR. LUBBOCK ON THE TIDES. 
219 
preceding , still this is not the case actually, and no approximation even would be 
obtained to the true parallax or calendar-month inequality in this manner. Nor did 
Bernoulli indicate the great difference in the retard, or age of the tide, at different 
places ; and he appears to have attributed this retard* to the inertia of the water, an 
error which Laplace pointed out. The difficulty to which I have alluded in ascer- 
taining the correct interval between a given transit of the moon and the time of high 
water does not influence so much the calculation of the heights, because the parallax 
and declination corrections for the height change very little with the moon’s age. 
In forming future discussions similar to that contained in this paper, it is desirable 
that they should be instituted with reference to the same transit of the moon, namely, 
with reference to the transit which precedes the time of high water at London by 
about 5 1 hours ; otherwise even the variations in the heights will not be immediately 
comparable with those here given, and the variations in the intervals will be very 
different. This may be seen by comparing the tables in this paper with those which 
I obtained formerly with Mr. Dessiou’s assistance, and in which the discussion of 
the same observations was instituted with reference to the transit immediately pre- 
ceding the time of high water. The variations in the interval between two successive 
transits of the moon are, in fact, of the same order in amount as those in the interval 
between the moon’s transit and the time of high water due to the variations in mag- 
nitude of the attractive forces ; and when the interval between the time of high water 
and the moon’s transit immediately preceding is considered, (at least on our coasts,) 
the variations from both these causes are mixed up together. 
As the tide-wave travels northward to the coasts of Great Britain from the Cape 
of Good Hope, passing the French coast, the variations in the interval and in the 
height at Brest must be similar to those at London and Liverpool. My results ought, 
therefore, to agree with those which may hereafter be deduced from the observations 
made at Brest by order of the French Government, and not yet published. The Brest 
observations may, however, be rather more accurate than those to which I have had 
access; and as the tide is single, the diurnal inequality is perhaps there more 
distinct. 
Although the imperfection of observations renders it indispensable to employ the 
average of a great number in order to deduce with safety any conclusions, this is 
equally required on account of the influence of what may be termed accidents, such 
as the winds and the varying pressure of the atmospheric column. M. Daussy has 
* “ Nous avons encore fait voir, que sans le concours des causes secondes les plus grandes marees devroient 
se faire dans les syzygies et les plus petites dans les quadratures. Cependant on a observe, que les unes et les 
autres se font un ou deux jours plus tard. Ce retardement est encore produit, si non pour le tout, au moins 
en partie par l’inertie des eaux qui doivent etre mises en mouvement et qui ne sauroient obeir assez promte- 
ment aux forces qui les sollicitent, pour leur faire suivre les loix que ces forces demanderoient.”— p. 158. I use 
the word retard after Bernoulli. 
t This is also desirable with reference even to the establishment of ports, for obvious reasons. 
2 F 2 
