Ue THE ROYAL SOCIETY OF CANADA 
the main length of an inlet running in from the ocean. In an ordinary 
estuary, which is shallow relatively to its width, the tidal undulation 
proceeds along it as a wave of translation; but if the depth is great 
(say over 100 fathoms) and the width fairly uniform, the tidal impulse 
at the mouth causes the whole surface of the inlet to rise and fall 
simultaneously, without appreciable current. There are many such 
inlets in British Columbia. (See explanations in ‘‘Progress of the Tide 
in Deep Inlets and Ordinary Estuaries,’ by the present writer.) 
The following classification may therefore be made:— (1) Ordi- 
nary estuaries; omitting deep inlets of uniform width from the mouth 
to the head, in which the current is usually inappreciable. (2) Straits 
of moderate depth; as there are some so-called straits with a continu- 
ous depth of 100 to 200 fathoms which may show the characteristics 
of the deep inlets. (3) Large basins or expanded inlets, connected 
with the ocean by narrow entrances; their area being so large that there 
is not time for them to fill up during the tidal period. This type of 
inlet is more common than might be supposed. 
Types of tide—The influence of the moon is exerted in three 
leading ways, in the periods of the three ‘“‘months”’ in which the moon’s 
motions take place. Although these influences are always present in 
some degree, the noteworthy fact is, that in any region some one of 
these influences is usually so dominant as to give a distinctive character 
to the tide. There are thus three types of tide-—(1) The Synodic, 
when the variation from springs to neaps, in accord with the moon’s 
phases, is the predominant feature. (2) The Anomalistic, when the 
greatest variation is in accord with the moon’s distance from perigee 
to apogee. (3) The Declination type, characterized by diurnal 
inequality which is so pronounced as to be the leading variation. 
In dealing with time values, it is the declination influence which 
gives rise to the most striking variations. It is quite common for 
the time-interval to show an alternation of a full hour, on successive 
tides or slacks, when the moon is in high declination. 
Estuaries—In ordinary estuaries, in which the tidal undulation 
runs evenly, it is usual for the current to turn at a definite interval of 
time after high or low water. To obtain a constant time-relation, 
the chief proviso is that the tidal stations to which reference is made, 
are suitably situated in the estuary. The St. Lawrence estuary, 
320 miles from its mouth to the head of tide water, affords a large- 
scale example; and the Bay of Fundy may also be classed as an estuary 
from the present standpoint. Estuaries may be passed by with these 
remarks, as they are fairly well understood and considerable publi- 
cation exists regarding them. 
