76 THE ROYAL SOCIETY OF CANADA 
which make up the lunar day of 2434 hours, are as follows:—At one 
station, 1134 and 13 hours; at the other, 9 and 1534 hours. The 
mid-time between high water at these two stations was therefore 
taken for comparison with low-water slack; and the result was a 
remarkably constant difference, though large in amount. A fuller 
explanation of this research would be of much interest. 
Concluding -notes—The object in view is to obtain a constant 
difference between the time of high or low water, and the time when a 
tidal stream reverses its direction; or the time when it attains its 
maximum velocity. The difference may be considered as constant 
if the variation from the general average for the month or the year is 
within admissible limits. To supplement the explanations already 
made, the following points may be noted :— 
(1) When the current is out of accord with the local tide, it 
may correspond with the tide of the open ocean beyond the local 
channels. This is especially likely in a strait or passage behind an 
island, where the tide comes round both ends of the island. 
(2) When the current is sufficiently moderate for navigation to 
be possible at all stages of the tide, the time of maximum velocity 
may serve for practical purposes; as it may have a more constant 
relation to the time of the tide than slack water has. The time of 
the maximum can best be ascertained by current-meter observations. 
(3) To obtain the differences with the time of the tide which are 
most nearly constant, it may be necessary to distinguish the two 
slack waters, or the maximum velocity on the flood and ebb respec- 
tively; and to deal with these separately, with reference to two different 
tide stations in opposite directions. 
(4) When slack water cannot be brought into a constant aies 
with the tide at any station that can be found, a system of variable 
differences must be resorted to. It may be one only of the two slack 
waters that this is required for. 
For calculation purposes, a series of variable differences may give 
quite as accurate results as a constant difference does in other cases; 
as any variation from the average always recurs in some astronomical 
period, and the variable series represents its reduction to law. The 
use of such a series entails the publication of tables of Slack Water 
in full; whereas if a constant difference can be arrived at, the mariner 
can apply this for himself to the tide tables indicated. 
