LXXX ROYAL SOCIETY OF CANADA 



The extreme tides of the year necessarily occur at the nearest point 

 to the solstices at which the moon reaches its maximum declination. 



A tide of this character is apt to be termed irregular by the 

 mariner; as the tropical or declination-month which is its governing 

 period is less familiar and less noticeable than the synodic month of the 

 moon's phases. It is evident, however, that this tide is perfectly as- 

 tronomical; and its analysis and prediction are just as definite as for 

 any other type of tide. 



Mean Sea Level. — ^With a tide of this type, there is a notable differ- 

 ence betweeiu the half-tide level, and the true value of mean sea level. 

 These may diifer as much as a foot from each other, even in the case of 

 a tide wlhose extreme range is only 13 feet. The only accurate value 

 therefore, is the mean ordinate found by the integration of the tide 

 curve, referred to any invariable base line or datum. This mean or- 

 dinate fixes the position of the horizontal line which bisects the area of 

 the tide curve; and this also accords with the best definition of mean 

 sea level for any type of tide. 



The advantage of a registering tide gauge is much emphasised, 

 with tides of this character. If scale readings are taken by direct ob- 

 servation, which the Admiralty surveyors usually prefer, they must 

 be continuous, day and night, and afterwards plotted as a curve ; or little 

 use can be made of them except for the reduction of soundings. A 

 registering tide gauge not only gives the hourly ordinales of the tida 

 curve throughout the year as a basis for mean sea level, but the con- 

 tinuity of the record enables the diurnal inequality to be followed. 

 When this is known, the average and extreme levels of high and low 

 water, and questions of range, can be determined without fear of in- 

 accuracy. The continuous record is equally important with respect 

 to the time of the tide, in which there is a similar inequality of interval. 



TIhe question of mean sea level is of unusual interest on the Pacific 

 coast, as there is reason to believe that its elevation is changing. Some 

 indications poiut to a rise in the level of the coast, at as high a rate 

 as one or two feet per century. It is only from tidal observations pro- 

 perly reduced, that any trustworthy result can be arrived at; and if 

 the change is as rapid as supposed, it will not require an interval of 

 many years to obtain a fair approximation to its amount. 



