9 
]\J R. AIRY ON THE LAWS OF THE RISE AND 
high and low water, for the purpose of ascertaining the durations of the rise and fall 
of the tide. With this view, the depression of the water below the wharf was ob- 
served at the four quarters-of-hour nearest to the time of high water, and at the 
seven quarters-of-hour nearest to the time of low water (a greater number of obser- 
vations being made near low water, because less is known, from other observations, 
of the time of low water). And 1 proposed, by combining the observations of each 
group, to find the times of high water and of low water much more accurately than 
by the rude observation of the highest or the lowest water. But in discussing the 
low-water observations, I found an unexpected difficulty. The rise of the water at a 
given interval after low water (in half an hour, for instance) is considerably more 
rapid than its descent at the same interval before low water. There is in fact the ru- 
diment of a veritable bore. It is impossible here to use any observations for deter- 
mining the time of low water except those which are very near to the low water. 
My curiosity was now excited to learn with a little more precision the laws of the 
rise and fall of the water generally. For this purpose, Captain Shirreff undertook, 
at my request, to arrange for the observation of the depression of the water at every 
quarter-of-an-hour, night and day, during half a lunation. This was done, so far as 
I can judge, with the most perfect regularity. The observations commenced at Fe- 
bruary 16, 12 h 15 m , astronomical time, and finished at March 4, 12 h 0 m . The whole 
number of observations is 1536. 
In the computation of these observations it is to be considered that the times of 
high and low water are subject to perpetual irregularities, as well from the change of 
conformation of the sun and moon, as from the effects of the wind : and also that the 
heights are more conspicuously variable from the same cause. But there is this dif- 
ference between them, as regards the mode of treating the observations on the pre- 
sent occasion. The times of high water at London are predicted with considerable 
accuracy in the Nautical Almanac : and though the time of high water at Deptford, 
even when undisturbed, is not the same as at London, yet its difference may be sup- 
posed to be nearly constant, and therefore the time of high water at London will be 
a proper zero of phase to which to refer the Deptford tides. But there is no predic- 
tion whatever of the depressions of the Deptford high and low waters ; and therefore 
it appears impracticable to take any zero except the observed least and greatest de- 
pressions. These considerations suggested the following methods of reducing the 
observations : — 
The time, between one predicted high water for London and the next, was sup- 
posed to correspond to 360° of phase ; and the interval of each time of Deptford ob- 
servation from the preceding London predicted time of high water was converted 
into phase by that proportion. 
The space, between the least depression and greatest depression in one semi-tide 
(rise or fall), was supposed to correspond to 2, or the double radius ; and the 
depression of the water at each Deptford observation below the least depression of 
that semi-tide was converted into parts of 1 or radius by that proportion. 
