128 THE ROYAL SOCIETY OF CANADA 
types of the tide on the Atlantic and Pacific coasts. It has been found 
as a rule that the variation in some one period is so largely dominant 
that all others may be neglected. 
The variation usually takes place in one of the three lunar months, 
the synodic, the tropical or the anomalistic. The variation in the 
tropical month with the moon’s declination is by far the most common; 
as differences which remain constant during the greater part of the 
month, show a marked alternation in value with the moon’s upper and 
lower transits for a few days near maximum declination. In other 
regions, notably on the Pacific coast, the annual variation is so dominant 
that any monthly variation in the tidal difference is negligible. The 
same variations obtain in the relation between the turn of the current, 
or slack water, and the time of high and low water. A fuller discussion 
of this question will be found in “ Variation in the Leading Features 
of the Tide in Different Regions.” (Journal, Royal Astronomical 
Society of Canada, 1907, pages 213 to 227.) 
By the use of variable differences, the region referred to each tidal 
station may be extended; thus avoiding the multiplication of principal 
stations for which tide tables must be calculated astronomically. 
Variable differences are similarly used for the computation of slack-water 
tables from the primary tide tables. 
