234 



HYDROGRAPHICAL SURVEYING [chap. ix. 



Determi- 

 nation of 

 the Es- 

 tablish- 

 ment in 

 the Case 

 of Large 

 Diurnal 

 In- 

 equality 

 in the 

 Times. 



Approxi- 

 mate Pre 

 diction of 

 Tides by 

 Means of 

 Tidal 

 Diagram. 



tide in twenty-four hours. This generally happens only for a 

 few days in each semi-lunation ; at other times there are two 

 tides, as usual. 



In some places the tide rises and falls four times in the twenty- 

 four hours ; these may be called double half-day tides. They 

 do not commonly extend over any considerable length of coast. 



A notable feature is the sequence in which the higher and 

 lower low waters occur relatively to the higher and lower high 

 waters. The connection between the moon's parallax and the 

 range of tide is usually very characteristic, especially in the 

 Pacific amongst the islands. The highest tides generally occur 

 at equinoctial springs, when perigee occurs towards new or 

 full moon. It sometimes happens, as in the Bay of Fundy, 

 that the variation in range from perigee to apogee is greatei 

 than the difference in range at mean springs and mean neaps. 

 All over the Pacific the leading feature of the tides is a pro- 

 nounced diurnal inequality in time and height, which accords 

 with the declination of the moon ; and it is also subject to an 

 annual variation with the change in the declination of the sun. 

 When the moon is farthest north or south of the Equator, the 

 inequality between the two tides of the day is greatest. The 

 extreme tides of the year necessarily occur at the nearest 

 point to the solstices at which the moon reaches its maximum 

 declination. On the other hand, the tides become equal when 

 both the sun and moon are on the Equator, or when they are 

 on opposite sides of the Equator at distances north and south 

 which are proportional to their respective effects. 



When the diurnal inequality in the times is very perceptible, 

 allowance is made for it by drawing a curve, cutting off equal 

 portions above and below the zigzags formed by joining the 

 points denoting the lunitidal intervals for successive high 

 waters. This 7nean line will be of a wavy form in consequence 

 of the semi-diurnal inequality ; and the ordinate corresponding 

 to the new or full moon, or to the hours or 12 of moon's 

 transit, will give the establishment. But if we apply this estab- 

 lishment to predict the time of tide on any day, we must 

 remember that the diurnal inequality will affect it. 



To predict the tide for any particular day, if the diagram 

 extends over a considerable period, conditions as similar as 

 possible to those existing on that day as regards sun and 



