[pawson] METHODS OF INVESTIGATION OF TIDES & CURRENTS 5 
up to 1893, when the writer was appointed to organize a survey to 
carry on the work systematically. 
Tides.—Under the circumstances, there was practically a clear 
field to work upon. The tides themselves were known to be very 
complex in character, as they ranged from almost nothing in the middle 
of the Gulf of St. Lawrence to the highest in the world in the Bay of 
Fundy. The general method adopted from the outset was to establish 
principal stations at strategic points, to which the harbours in the 
surrounding regions could be referred. It was the intention from the 
beginning that the principal stations should be adequately equipped 
to obtain tidal record which could be reduced by the modern method 
of Harmonic Analysis. For this purpose it is essential to have con- 
tinuous record day and night throughout the year. There were thus 
many practical difficulties to be overcome; such as the heating of the 
tide gauges in winter by a method devised for the earliest stations, 
construction problems in the establishment of gauges where no artificial 
structures existed and the exposure was severe, and the design of an 
unfailing recording instrument for situations where no repair could 
be obtained. In such matters little help could be found in anything 
previously done in other countries. 
The two essentials for tidal observations are correct time and some 
datum or plane of reference from which the height can be measured. 
At some of the tidal stations the time can be readily ascertained, but 
at others it has to be specially obtained either by telegraphic signals 
or by meridian instruments which determine the time directly from 
the sun. At almost all the tidal stations it has been necessary to 
originate a datum level for height, and to establish a bench mark. A 
tide scale is set with reference to this bench mark, to maintain a uni- 
form datum throughout the years of observation. As an open scale 
becomes useless in the winter season, on account of the accumulation 
of ice, it was necessary to devise special apparatus which can be enclosed 
and protected. 
With regard to the limits of accuracy, it is sufficient to have the 
time correct within one minute, as this is as close as individual readings 
can be taken on the tide diagrams. The limit of accuracy for height 
is in general 0.01 foot. 
The equipment of the tidal stations to secure a continuous record, 
summer and winter, for harmonic analysis, and to obtain time and 
height for the observations under conditions of isolation and winter 
ice, are described in a Paper communicated to the Institution of Civil 
Engineers of London (2). 
Currents.—In the investigation of the currents the advantage of 
current meters registering electrically was very evident; but at that 
