102 



period of the heights of high and low waters above arbitrarily selected 

 zero elevations at the two stations. These heights are called the 

 respective high and low waters on the staff. The mean high and low 

 waters, and the half-tide levels on the staff at the two stations during 

 the period of observation are computed. The difference between the 

 established half-tide level and the observed half-tide level at the pri- 

 mary or base station gives the correction to be applied to the observed 

 half-tide level at the secondary station. 



198. Mean sea level by comparison.^-li the base and secondary sta- 

 tions both have a free connection with the sea or are freely connected 

 with each other by deep water, so that they both may be presumed to 

 have the same overtides, the difference between mean sea level and 

 half-tide level should be the same at both. This difference, as deter- 

 mined at the base station, applied to the corrected half- tide level at 

 the secondary station, gives mean-tide level at the secondary station. 



199. Mean high and low waters by comparison. — Wliile the tidal 

 range often varies materially from station to station in the same region, 

 the heights of the successive low and high waters with respect to half- 

 tide level at one station are proportional to those at another if the 

 amplitudes of the components of the tide at one of the two stations 

 have a constant ratio to those at the other, and the epochs of the 

 several components at one station differ from those at the other by 

 a constant angle. These conditions are to be expected when the tides 

 at the two stations are both produced by the same offshore fluctua- 

 tions of the ocean. They are exemplified by the relationship of the 

 principal tidal components at stations on the New England coast 

 north of Cape Cod. Harmonic constants have been determined at 

 Portland, Maine, at Pulpit Harbor, 80 miles to the northeast, at East- 

 port, 190 miles northeast, and at Boston, 90 miles to the south of Port- 

 land. The ratio of the amplitudes of the principal tidal components at 

 these stations to those at Portland, and the difference in the epochs 

 of the respective components, are shown in the following tabulation, 

 prepared from the data set forth in table V, paragraph 134. To 

 extend the comparison, the ratios of the amplitudes of the principal 

 components at Fernandina, Fla., 1,000 miles to the southward, to 

 those at Portland are added, together with the difference in their 

 epochs. 



