224 MR. ROBERT GURNEY ON THE TIDES OF 
Similarly at Acle Bridge the average duration of the rise 
from a week’s observations was found to be 5 h. 36 m. and 
of the fall 6 h. 40 m. The difference in duration is not constant, 
since it is affected by wind and other factors. For instance, 
at Thurne Mouth at neap tides and with a moderate Westerly 
wind the rise and fall took approximately the same time — 
6 h. and 6 h. 15 m. respectively, while, at the same place on 
another occasion at spring tides and with light Southerly 
winds following on Northerly winds, the rise lasted 5 h. 40 m. 
and the fall 6 h. 35 m. 
The height of the wave decreases as it proceeds up the river, 
and the abrupt rise at the beginning of the flood becomes 
less and less pronounced. As a consequence of this the curve 
traced by the tide gauge becomes flatter and more even, 
this flatness making it exceedingly difficult and often impossible 
to decide the precise moment of high water. The tendency 
to a sudden rise from low water persists and generally permits 
the moment of low water to be ascertained fairly exactly. 
But at high water the level tends to remain stationary for 
a considerable period, sometimes for nearly an hour, and then 
ensues a rather sudden drop somewhat comparable to the 
sudden rise from low water in the lower reaches of the river. 
This change in the form of the wave can be well seen in the 
diagram (Fig. 2). But wherever a good normal tide curve 
can be obtained, the rise and fall are always greatest at the 
beginning of the flood and the ebb. 
The form of the tide wave appears to be very different in 
the Yare and the Waveney from what it is in the Bure. At 
Oulton Broad tracings were taken for six complete tides from 
September 21st to 23rd, 1906, and in each case the curve 
shows an absolutely simple rise and fall with no apparent 
difference between the beginning and the end of the flood 
(Fig. 3). Charts taken at Brundall on the Yare on June 22nd, 
1906, show much the same thing, but in this case there is 
a distinct trace of a bore at the beginning of the flood, and 
so an approximation to the conditions in the Bure. I cannot 
offer any explanation of this difference unless it is to be 
accounted for by the greater depth and volume of these rivers 
offering less obstruction to the progress of the wave. 
