[DAWSON] THE TURN OF TIDAL STREAMS 73 
Straits —The complication in straits, as distinguished from estu- 
aries, generally results from tidal undulations entering at both ends. 
In longer straits, this occasions complication from interference. 
In shorter straits, the flow is more directly the result of a difference of 
water-level, caused by difference in the time of arrival and the range of 
the tide at the two ends. It is remarkable to find, however, that all 
these proximate influences may be ignored; and the behaviour of the 
current brought into direct relation with the moon’s movements, 
as their primary cause. 
We may. begin with straits in which the current, like the tide 
itself, is under the dominant influence of the moon’s declination. 
Northumberland strait affords the best example, as principles dis- 
covered elsewhere were applied to it; and also the time of maximum 
current was accurately ascertained in each of its three narrows, by 
current-meter observations. In such a strait, when the moon is in 
high declination, the turn of the current is alternately earlier and later 
than the average, in relation to the time of high and low water; and 
the strength of one flood and one ebb in the day is much greater than 
the strength of the other two. To reduce the variations in the time- 
intervals to law, it was found necessary to classify the tides themselves, 
with distinction of the moon’s transits. The high or low waters 
which follow the moon’s Upper transit when in North declination, 
and those which follow the moon’s Lower transit when in South declina- 
tion, are termed “Similar.” The remaining high or low waters, 
which follow the moon’s Lower transit when North or the Upper 
transit when South, are termed “‘Opposite.’’ It was also discovered 
that in Northumberland strait the flood is related to the tidal station 
in the entrance to the Gulf of St. Lawrence where the tide comes in 
from the ocean, and the ebb to the tidal station on the inland side of 
the Gulf, in the St. Lawrence estuary. 
These principles of classification and distinction, have the widest 
application in dealing with time-differences which show a marked 
alternation when the moon is in high declination. In such cases it is 
also usual to find an appreciable annual variation with the sun’s 
declination. 
These methods have been fully elaborated in dealing with the 
time-differences between tide and slack water, for the calculation of 
Slack-water tables for Seymour Narrows on the Pacific coast. The 
series of alternating values resulting, has been further checked and 
revised by observations of slack water during a season in the 19-year 
cycle when the moon’s range in declination was greatest. A full 
explanation of the successive approximations during recent years, in 
