[pawson] METHODS OF INVESTIGATION OF TIDES & CURRENTS 15 
had been adopted for each new purpose as it arose. The datum for 
city works had been altered more than once. The Royal Engineers 
and the Public Works department had independent levels of their 
own. Another datum was used for the harbour chart, and so on. 
When the tidal observations were begun, much trouble was taken by 
the writer to correlate these various levels and to adopt a satisfactory 
plane of reference for the future. For this purpose, instrumental 
levels were carried to Esquimalt, as some datum planes were better 
defined there than at Victoria. The relation between all existing planes 
of reference was eventually determined and brought into relation with 
the tidal observations. 
The results for Victoria, Vancouver and other cities and towns 
of British Columbia, are given in a Paper, prepared by the writer, which 
was published by the Marine department, entitled “Tide Levels and 
Datum Planes on the Pacific Coast” (21). This paper contains a complete 
list of the Bench Marks which define datum planes in the cities, and 
also those which define the low water datum of the charts at all points 
along the coast where Admiralty surveys have been made. 
Importance of Bench Marks.—The importance of establishing 
Bench Marks in connection with tidal observations is very evident, 
as it is only from these observations that data for many purposes 
can be obtained. The extreme levels of high and low water are im- 
portant for wharf construction and docks, and also for sewerage and 
other city works. They are also essential for dredging operations in 
the deepening of harbours and channels. The value of mean sea level 
is very accurately determined by the continuous observations required 
for harmonic analysis; and where there isa Bench Mark for reference, 
the level is permanently fixed. This determination in our principal 
harbours affords a basis for geodetic levelling; and it is also utilized 
by the Public Works department, the Interior department and the 
Geological Survey, as well as for our principal canals. Such determin- 
ations of mean sea level afford the only means by which an alteration 
in the level of the continents relatively to the ocean can be detected. 
In several regions such gradual changes are taking place, and they 
have a practical bearing on the depth of harbours and channels. In 
such ways as these, the levels as determined by the Tidal Survey have 
an extended value; and they will become increasingly useful for refer- 
ence in the future as the country develops and a wider accuracy is 
required. 

