164 
THE REV. W. WHEWELL’S RESEARCHES ON THE TIDES. 
greater degree, the heights of the low waters. And thus some of the high waters are 
depressed, and some of the low waters elevated, till there is little vertical distance 
between the two. On the 17 th of June the rise from low to high water in the afternoon 
was only eight inches, although the rise in the forenoon had been four feet. In the 
same manner, the fall from high to low water in the forenoon of June 22 was only 
two inches, and in the forenoon of June 23 only one inch, although the intermediate 
fall in the afternoon of those days was above four feet. And these movements of the 
surface in the forenoons of those days, which we have called a fall from high to low 
water, are in fact more simply conceived as a mere temporary check in the rise of 
the water from the preceding low to the succeeding high water. And accordingly 
on June 24 the movement plainly assumes this character ; the ascent is checked, but 
no descent takes place. In the register of the observations it is stated that “from 
6 h to 8 h the surface remained stationary, so that we may say there was no low 
water.” And thus one of the two half- daily tides being obliterated, we have at this 
period only one tide in the twenty-four hours. 
This phenomenon, or an approximation to it, has already come under our notice 
in the tides of Sincapore, of the Keeling Islands, and of Bassadore ; and has been 
noticed by others in other places. It is to be recollected, however, that this reduc- 
tion of the two half-day tides to a single-day tide takes place only at a particular 
period of each lunation, depending upon the declination of the moon. When there- 
fore a traveller has such a phenomenon brought under his notice, he should recollect 
that by pursuing his tide observations for a few days with assiduity, he will find the 
single-day tide resolve itself into the usual case of two daily tides. 
“ The establishment ” ceases to be applicable . — The diurnal inequality affects the time 
of high water at Petropaulofsk to a very large amount, as we have seen ; and hence 
the interval of moon’s transit and high water, whether taken at the syzygy or at any 
other time, may vary very much from one tide to another. I have already noticed 
cases in which these intervals were alternately l h and 5 1 ', 2 h and 6 h , 3 h and 7 h - Now 
this being the case, what do we mean by the Establishment of the place ? Supposing 
such observations as those just mentioned to be made at the syzygy, which of the 
two numbers of hours is to be taken as the establishment, in the common sense of the 
term ? There is no ground for preferring either to the other, and therefore we can- 
not with any propriety deduce the establishment from either. 
Perhaps it will be said that we may take the mean of two, or of an even number of 
successive high waters; and thus obtain an approximation to the establishment, such 
as it would be if not affected by the diurnal inequality. But even if a mean esta- 
blishment were so obtained, it could not be applied. Thus if it appeared that the 
lunitidal interval for the time of syzygy, cleared of diurnal inequality, were five hours, 
still, as the cycle of the diurnal inequality is entirely different from the time between 
syzygy and syzygy, the interval at syzygy may not be affected at all by the inequality, 
or it may be affected by the greatest positive, or by the greatest negative value. It 
