TIDES AND CURRENTS IN BOSTON HARBOR 13 



high and low waters for 30 years (1847-1876), are 11.09 and 8.32 feet, 

 respectively. The perigean and apogean ranges, as derived by apply- 

 ing to the mean range differences obtained from the harmonic anal- 

 ysis of the hourly heights for the year 1869, are 11.37 and 8.39 feet, 

 respectively. 



It will be noted that the perigean range is a little greater than 

 the spring range. In this locality the parallax of the moon appears 

 to have a little greater influence in increasing the range of tide than 

 does the phase of the moon. When the perigean and spring tides 

 occur about the same time of month the range of tide will be es- 

 pecially large, and when the apogean and neap tides occur together 

 the range will be especially small. 



Diurnal ranges. — There is usually a difference in the height of the 

 two high waters of each day and also in the height of the two low 

 waters. For distinction, the two high waters of each day are desig- 

 nated as the higher high and lower high and the two low waters as the 

 higher low and the lower low. The great diurnal range of tide is the 

 difference in height between the average of all the higher high waters 

 and the average of all the lower low waters. The small diurnal range 

 is the difference in height between the average of all the lower high 

 waters and the average of all the higher low waters. 



The diurnal ranges for the navy yard, obtained by applying inequali- 

 ties from the reductions of a single year (1869) to the mean range for 

 39 years, are 10.44 feet for the great diurnal range and 8.98 feet for the 

 small diurnal range. 



Tropic ranges. — The high and low waters near the times of maximum 

 declination of the moon are designated as tropic tides. At these times, 

 while the average semidiurnal range of tide is less than usual, the 

 inequality in the heights is generally greater. The great tropic range 

 is the difference in height between the tropic higher high water and 

 the tropic lower low water, and the small tropic range is the difference 

 in height between the tropic lower high water and the tropic higher low 

 water. The values of these ranges, as derived from an analysis of the 

 observations for the year 1869, are 10.24 feet for the great tropic range 

 and 7.85 feet for the small tropic range. 



Extreme ranges. — Extreme tides result primarily from meteorologi- 

 cal conditions, but they may also depend largely upon combinations of 

 astronomical causes. The highest and lowest tides for each month of 

 observations at the navy yard, as far as available, are given in Tables 5 

 and 6. The average of the monthly extremes for each year and the 

 averages for the corresponding calendar months throughout the series 

 are also given. The average monthly extreme range of tide is the dif- 

 ference between the average of the monthly extreme high waters and 

 the average of the monthly extreme low waters. For the navy yard 

 this was found to be 13.93 feet. The greatest range between the high- 

 est tide observed and the lowest tide observed was 18.5 feet. 



Diurnal inequalities. — The diurnal high-water inequahty is the dif- 

 ference between the mean of the higher high waters and the mean of 

 all high waters. The diurnal low-water inequality is the difference 

 between the mean of all low waters and the mean of the lower low 

 waters. As determined from the high and low waters for one year 

 (1869), the diurnal high-water inequality at the navy yard was found 

 to be 0.40 foot and the diurnal low-water inequality as 0.33 foot. 

 81772—28 2 



