DR. HAROLD JEFFREYS ON TIDAL FRICTION IN SHALLOW SEAS. 
259 
it is at llh. 18m., and the mean of the times at Westport, Petit Passage, and Digby 
Gut, which are nearly opposite, is lOh. 48m. Thus the mean time of high water in 
the channel must be about llh. 3m., so that the current turns 53 minutes after the 
tide. The phase difference is therefore 25 degrees. The width of the channel is 
83 km. Applying equation 15 of Taylor’s paper, we find that the average rate at 
which energy enters is 47 x 10 17 ergs per second. 
The average rate at which the moon does work on the bay is —\g j hh'y sin /3c£S, 
as was found for the Yellow Sea. In this case the latitude is 45° north, so that h! is 
18 cm. In the lower half of the bay the amplitude of the tide is not greater than 
12 feet, but in the upper half it rapidly increases, till in Minas Basin it reaches 25 feet 
and in Chignecto Bay 23 feet. The time of high water in the bay ranges from 
llh. 3m. to llh. 50m. The later value corresponds to the upper part, where the 
amplitude is greatest; but as this part is also the narrowest, the two times must 
receive about equal weights in finding the average. We therefore take the average 
time of high water to be llh. 30m. The average amplitude of the tide is about 
18 feet, or 540 cm. The longitude of the bay is 66° west, so that the moon crosses 
the meridian at full and change at 4h. 33m. The time of high water is more than 
6h. 12m. later than this, so that the tide is falling when the moon is exerting its 
greatest upward pull, and the work done by the moon is therefore negative. The 
interval between transit and low water is 45 minutes, so that /3 = 22°. The area of 
the bay is l'2x 10 14 sq. cm. The work done by the moon is therefore — 3 x 10 18 ergs 
per second. The total dissipation in the bay is 4'4 x 40 17 ergs per second. 
This estimate is six times as great as the earlier one based on the currents alone. 
It is much the more reliable, for the first depended on the assumption that the 
currents were equally great all the way up the bay, whereas actually they increase 
very much towards the head. Velocities up to 9 knots are recorded in Minas Basin, 
though the area in which these occur must be very restricted. The most serious 
source of error in the second estimate is the phase difference, for this is only an hour 
and would be affected to a considerable extent by an error in the determination of 
the time when the current turns. The observed time of turn does not vary much 
from place to place, however, and it does not seem likely that the estimate is wrong 
by more than a quarter of its amount. The second estimate will therefore be 
adopted. It will be noticed that it is rather less than the dissipation in the Irish 
Sea. 
The Gulf of St. Lawrence gives very little dissipation. The narrow entrance 
through Cabot Strait prevents the tides from being considerable except in the 
estuary of the river itself and in Belle Island Strait, which separates Newfoundland 
from Labrador. 
