242 MR. ROBERT GURNEY ON THE TIDES OF 
was 705 gr. per gallon-undoubtedly a fall from what it had been. 
On that day I left Stokesby, and found that even then the 
salinity at Thurne Mouth showed a strong admixture of salt 
water, though at that point fresh-water species reappeared. 
The records of the tides at Yarmouth show that the morning 
tide of September 3rd ran up to 7 ft. 7 in., or 2 ft. 6 in. higher 
than the previous one, but that at 3.20 a.m. on September 4th, 
low water stood at 2 ft. 6 in., and in five hours the tide rose 
to 8 ft. 3 in. It was no doubt this tide that drove such an 
immense body of salt water up the river. It is, however, 
possible that the previous tide sent some up, and that the 
water was not quite fresh when I was collecting fresh-water 
species, but there must have been a very great change between 
noon on September 3rd and noon on September 4th. The 
change is all the more striking from the fact that on September 
14th conditions were so entirely reversed that the water was 
entirely fresh right down to the Yacht Quay at Yarmouth. 
A tide of exactly the same height occurred on July 7th, 1903, 
and then marine species were still to be found at Thurne 
Mouth on July 13th, and the salinity even at Sutton was 
above the normal. 
The highest tides that have been recorded at Yarmouth 
have occurred during the winter, high summer tides, so far 
as I can find, never reaching much above 8 feet. There is 
reason to believe that the effect of the high winter tides does 
not extend further up the river than that of the summer 
tides ; but, as the water level is always higher in winter than 
in summer, it seems that the amount of fresh water in the 
river determines the limit of influence of the salt water. It 
is, therefore, to be expected that during winter the point to 
which salt water can normally penetrate will be lower down 
than in summer. 
While the effect of Northerly winds in causing exceptional 
tides is easy to demonstrate the influence of other winds is 
not quite so clear. It can be shown, however, that strong 
South-west winds have the reverse effects to those of North- 
west winds. For example, on February 10th, 1906, there was 
a moderate gale from the South-west, the wind during the 
