103 



It may be noted that the ratios of the amphtudes of the semidiurnal 

 components at Eastport to those at Portland are quite consistent, as 

 are the ratios of the diurnal components, but the ratios of the semi- 

 diurnal differ widely from the diurnal. At Fernandina a similar 

 divergence occurs in the differences of the epochs. 



200. Since the successive heights of high waters and low waters 

 with respect to half-tide level at one station are found to have a 

 substantially constant ratio to these heights at another station in 

 the same region, the long-term means of the high and low waters at 

 the two stations are proportional to the respective mean values 

 during any period of concurrent observations. The ratio of the 

 mean range at the primary station during a period of concurrent 

 observations to its established long-term mean, applied to the mean 

 range during the same period at the secondary station, gives therefore 

 the corrected mean range at the secondary station. The corrected 

 heights of mean high water and mean low water on the staff are then 

 obtained by adding and subtracting one-half of the corrected mean 

 range to the corrected height of half tide on the staff. 



201. It may be observed that a comparison, if based on a fairly long 

 set of concurrent observations, will give reliable results even when 

 the timing of the components is not the same at the two stations, 

 for, as shown in appendix II, mean range at each station depends on 

 the M2 component and the ratios of the other components thereto, 

 and not on the epochs of these components. 



202. Mean low water and mean high water of spring tides by com- 

 parison. — It has been shown (par. 184) that the spring range may be 

 taken as Mn+2S2. Since the amplitude, S2 ordinarily has a constant 

 ratio to the amplitudes of the other principal components at stations 

 in the same region, and hence to the respective mean ranges at these 

 stations, the ratio of the spring range to the mean range should be 

 the same at all such stations. After this ratio has been determined 

 at a base station, it may be applied to the corrected mean range at 

 any secondary station, as derived by comparison, to determine the 

 spring range at the secondary station. Mean low water of spring 

 tides at the secondary station is then one-half the spring range below 

 the corrected half-tide level, and mean high water of spring tides 

 one-half of the spring range above the corrected half- tide level. At 

 stations on the Pacific coast of the Panama Canal Zone, for example, 

 the ratio of spring range to mean range is 1.26, and the elevation of 

 low water of spring tides is taken as HTL— 0.63 Mn. 



203. Mean lower low and mean higher high waters by comparison. — 

 It has been seen that the elevation of mean higher high water exceeds 

 that of mean high water by the diurnal high water inequality, DHQ, 

 and the elevation of mean lower low water is that of mean low water 

 less the diurnal low water inequality, DLQ. The elevation of mean 



