Breaks in Tide Curves from Recording Gauges 75 



thus enable the variation during its course to be correctly determined 

 eventually, from observation alone. 



Examples of Interpolation. — The interpolation which we here give as 

 an example, is the filling of gaps which occurred in a tide of this declina- 

 tional type at its extreme accentuation in January and December near 

 the solstices; and if the method described will answer in such a case, it 

 may safely be taken as a guide for the interpolation of 'a break in the 

 continuity of tide curves of a less exacting character. 



In the Strait of Georgia, the tide is of this type. The curves in the 

 Plates will show its character. There were two gaps in the tidal record 

 from the principal tidal station in that strait during the year 1919; one 

 of nine days in January, and the other of ten days in December. Unless 

 these gaps could be filled, it is evident that at least the eleven months 

 intervening would be lost. If a break in a tidal record occurs when a year 

 is nearly completed, an overlap may be made to obtain 12 consecutive 

 months; but at this station, the complete years for analysis are now 

 running from May to May; and the overlap required would be too large. 

 With the best arrangement possible, the period lost would therefore be 

 be nineteen months; which shows what large losses may be involved if a 

 tidal record cannot be completed. 



Methods of Interpolation. — The breaks in the record, for which inter- 

 polation is required, may be classed as short and long. If not much over 

 a day, generally because of some slight accident, the missing portion can 

 readily be filled in as a rule, by superposing the curves for successive days 

 and interpolating between them. A difficulty only occurs when the 

 missing part happens to be the maximum or minimum at Spring or Neap 

 tides; or if the curves are disturbed from the normal before and after the 

 break. It is good practice also to replace any severe storm disturbance 

 by an interpolated curve. During a day, or a day and a half, the suc- 

 cessive tides may be raised or lowered from one to three feet; and such 

 abnormal curves would only deteriorate accuracy if included in the 

 analysis. 



The best method of dealing with long breaks, by which we mean a 

 week or ten days, is what we will now describe. It may be said in 

 general terms to be based on the recurrence of approximately the same 

 form of tide curve after the lapse of a month; and consequently, that it 

 should be possible to make an interpolation between the corresponding 

 days in the previous and following lunar months. The half-month will 

 not do, because the moon's distance varies during the period of a whole 

 month; and successive Spring tides thus usually show a "semi-monthly 

 inequality" which is always appreciable and in some regions is large. 

 But after the lapse of a month, the phases of the moon are not far out of 



