[DAWsoN] PROGRESS OF THE TIDE IN DEEP INLETS 27 
on so few miles of length every six hours, gives rise to a comparatively 
slight current which is found near the mouth only; as is verified by 
observation. 
As regards the ratio of the amplitude to the depth, it will be noticed 
in the cases cited that the spring range is from 1/40th to 1/60th of the 
total depth, as nearly as this can be estimated. 
Method of deducing Results.—In reducing observations to law, it 
is thus evident that a clear distinction should be made between ordinary 
shallow estuaries, in which the tidal undulation progresses as an advanc- 
ing wave, and estuaries or inlets of great depth, in which the rise and 
fall are practically simultaneous for long distances. 
The rate of progress is also subject to a variation which must be 
viewed from several standpoints. Although the wave progress in 
shallow estuaries is primarily a hydraulic problem, and must no doubt 
be some function of the amplitude, yet the amplitude itself is found 
to vary in the periods of the moon’s phases, its distance, and its declina- 
tion; these various elements being more or less influential in different 
regions. It is thus usually possible to bring the variation in the rate 
of progress into direct relation with the period of one or other of the 
lunar months. (See NoTE appended.) 
When, again, the estuary is moderately deep and sufficiently 
regular, the hydraulic problem may predominate; and the simpler 
method may prove to be the correlation of the rate of progress with 
the absolute height of each tide as observed on a scale at a reference 
station in the region, or as calculated in tide tables for that station. 
The Fraser river, as seen from the example given, is an estuary of the 
type to which this method is applicable. 
The limiting case is when the estuary or inlet is so deep that the 
amplitude of the tide becomes negligible relatively to the depth, and 
the time becomes simultaneous throughout the whole area. 
NOTE ON THE VARIATION OF TIDAL DIFFERENCES. 
The time of the tide for the harbours in any region is usually found 
by means of a difference of time relatively to a reference station in the 
region, for which tide tables are calculated. A constant value for the 
tidal difference, or time-interval relatively to the reference station, 
cannot always be accepted; as the value may vary too widely from 
the average. The manner of its variation in more extended regions 
than estuaries, has been a subject of careful investigation in Canadian 
waters. These afford an unusually good field for the research, owing 
to the complexities arising in land-locked areas; and the contrasting 
