8 
MR. AIRY ON THE LAWS OF THE TIDES IN THE RIVER THAMES. 
6th. This remark enables us to explain a circumstance which appeared somewhat 
perplexing. It has been found by Sir J. W. Lubbock and Mr. Whewell that the 
age of the tide is different, as inferred from the height of the high water, or from the 
time of high water : the age of the tide always appearing greater as inferred from the 
heights *. Now to explain this, we have to consider that, at syzygies of the sun and 
moon, the time of high water in the sea is on every successive day earlier with re- 
spect to the moon’s transit ; but the syzygial or spring tide in the river, and the tides 
near it, are (as we have just found) accelerated with respect to the sea-tide more 
than mean tides are : and therefore the river tide which happens at the mean inter- 
val from the moon’s transit is not the syzygial tide, but a tide preceding it. But there 
is no corresponding effect produced on the height of the tide. Thus the age of the 
tide inferred from the height is the true age (at least as far as it can be ascertained 
from the phenomena of the ocean-tides) ; the age as inferred from the time of high 
water is certainly too small, and the quantity by which it is too small depends on the 
length and depth of the river, or of the shallows along which the tide has to pass. 
I have only to add the following deductions from the observations. 
In division A (low tides) the high water occurred at the phase 350° nearly, that is, 
about 20 m before the predicted time of high water at London Bridge : the low water 
occurred at the phase 185° nearly: the interval between high water and low water 
was about 195° of phase, and that between low water and high water about 165° of 
phase ; or the descent occupied a longer time than the ascent by 30° of phase, or a 
little more than an hour of time. 
In division B (high tides) the high water occurred at the phase 355° nearly, or 
about 10 m before the predicted time of high water at London Bridge : the low water 
occurred at the phase 205° nearly; the descent therefore occupied 210° of phase, and 
the ascent 150°; or the time of descent exceeded that of ascent by 60° of phase, or 
2 h 4 m of time. 
The times of the turn of the tide-current, as shown by the swinging of the ships 
at anchor in the river, were regularly observed. The means of the corresponding 
phases in division A are 10 o, 4 and 204 o, 4, or nearly 20° of phase or 40 m of time after 
high and low water respectively: those in division B are 14 o- 0 and 223 0, 5, or nearly. 
18°*5 of phase or 37 m of time after high and low water respectively. 
* Lubbock, Philosophical Transactions, 1837. Whewell, Philosophical Transactions, 1838. 
Royal Observatory , Greenwich , 
June 25, 1841. 
