TIDES AND CURRENTS IN BOSTON HARBOR 17 



of the summer. Figure 5 shows the variation in the mean range, 

 which depends upon the relative variation in the high and low water 

 heights. This curve shows two maxima and two minima points. 



Mean tide level, which is a mean between the high and low water 

 heights, and the sea level, which is the average of the hourly heights, 

 are represented in Figure 6. It will be noted that these two curves 

 are approximately parallel and that the mean tide level is consistently 

 about a tenth of a foot lower than the sea level. Both the mean tide 

 level and the sea level are from two to three tenths of a foot higher in 

 the summer months than in the winter months. 



The fluctuations illustrated by these figures may be due partly to 

 meterological conditions depending upon changes in the seasons and 

 partly to astronomical conditions depending upon changes in the 

 declination and parallax of the sun. 



Intervals, ranges and inequaliiies. — In addition to the principal 

 tidal elements derived from the full series of observations, the sum- 

 maries of Tables 25 to 28 include the following quantities for Com- 

 monwealth Pier No. 5. Intervals, ranges, and tide planes depending 

 upon the tropic tides, and the ages of the tide were computed from 

 the harmonic constants given in Table 28. The spring, neap, perigean 

 and apogean ranges were obtained by applying differences derived 

 from the harmonic constants to the mean range of tide from the full 

 series of observations The diurnal high and low water inequalities 

 were obtained from three complete years of observations, 1922 to 1924, 

 inclusive. The diurnal ranges were obtained by applying the diurnal 

 inequalities to the mean range for the entire series of observations. 

 The relations of the spring and neap tide levels to the mean tide level 

 were obtained from a phase reduction of the high and low waters for 

 the year 1924. 



Tide planes. — The tide planes given in Table 27 are referred to 

 both the sea-level datum and the Boston low-water datum which 

 are defined on page 41. The relations of the different tide planes to 

 each other and to mean tide level depend largely upon the various 

 tidal ranges and inequalities. The mean tide level at the navy yard 

 was derived independently from the high and low water observations 

 covering some 39 years. The observations at Commonwealth Pier 

 No. 5 available at the time of this publication cover a period of only a 

 little more than five years. Because of the fluctuations in the yearly 

 mean level in any locality, this shorter period of observations is not 

 sufficient for a satisfactory independent determination of mean tide 

 level. Comparisons have therefore been made with simultaneous 

 observations at Portland, Me., where a mean level has been deter- 

 mined from 15 years of observations. During the period of obser- 

 vations at Commonwealth Pier No. 5 it appears that the average 

 level of the water was 0.07 foot below normal, and this difference has 

 been applied in Table 27 to the mean tide level and other tide planes 

 determined from the observations at Commonwealth Pier No. 5, 

 excepting the highest and lowest tides observed, which are taken 

 without modification. 



Harmonic constants. ^-The harmonic constants for Commonwealth 

 Pier No. 5, which are given in Table 28, were derived from an analysis 

 of two 369-day series beginning January 1, 1922, and January 1, 1924. 

 These constants are now being used in making the tidal predictions 

 for Boston, published in the annual tide tables of the Coast and 

 Geodetic Survey. 



