246 HYDROGRAPHICAL SURVEYING [chap. ix. 



others on the sun. Some occur once in tlie day, or are diurnal ; 

 others twice, or semi-diurnal ; some have a period of a lunar 

 montli dependent on the moon's position in her orbit as regards 

 the sun ; others have six montlily periods dependent on the 

 sun's declination. The moon's declination, a variable quantity 

 with a long period of years, controls others. The position of 

 the moon's node, and her varying distance from the earth, 

 are responsible for considerable waves, the perigee tide being 

 always larger than one in apogee. 



The system of harmonic analysis has been adopted for the 

 clearing of these dilferent waves, and is, when tides are very 

 variable, the only method by which the calculation and fore- 

 casting of tides is possible. 



It is not proposed to enter into this, which forms no part of 

 the necessary work of a marine surveyor in the field, and is a 

 subject in itself ; but it may be mentioned that while in a few 

 regions the time-honoured practice of calculating the time of 

 the tide from the moon's meridian passage, and its height 

 from the mean of tides observed in connection with that 

 meridian passage, may serve for practical purposes, in most 

 parts of the world the movement is so complicated that for a 

 satisfactory forecast the employment of harmonic analysis is 

 necessary. 

 General From what has been said it is evident that observation of 

 Remarks ^j^g ^^^g f^j, ^^ short period will, when tides are complicated, 

 afford no means of predicting them. The movement may be 

 wholly different when the sun is north of the Equator and 

 when it is south, and many other factors make it necessary to 

 observe for at least a year to gather an idea of the behaviour 

 of the tides at all seasons. 



The variety in complicated tides is infinite. In some cases 

 the water will rise to nearly the same level every tide, while 

 the low water shows great differences ; in others it is vice versa. 

 In some cases the diurnal inequality, the difference in height 

 of each successive tide, will be equally distributed between 

 both high and low water levels ; in others it affects only one, 

 or may vary with the season. 



One feature of a tide when there is much inequality is gener- 

 ally regular — i.e., the succession of the inequality in height. 

 At some places the higher high water is followed by a fall to 



