TIDES AND CURRENTS IN BOSTON HARBOR 15 



below the intake there is a supporting section 73^^ feet lon^ in which 

 iire cut four 3 by 9 inch slots for the admission of water. The intake 

 to the main pipe is about 7 feet below mean low water and 6 feet 

 above the bottom of the harbor. Kerosene is used inside the float 

 pipe to prevent freezing in the wintertime. Any difference in level in 

 the pipe due to the introduction of this kerosene is taken into account 

 in the tabulations by comparative readings on an exposed tide staff. 



Tide staff. — A portable tide staff was installed, the support being 

 secured to a pile on the north side of the pier near the inner end. 

 At first a 12-foot standard staff was used with the 12-foot mark cor- 

 responding to the top of support. On September 1, 1922, this was 

 replaced by a new staff, the support for which was nailed to the old 

 support with the top at the same elevation. The new portable staff 

 was set with the 15-foot mark corresponding to the top of this sup- 

 port. All tabulations of this series have been referred to the second 

 staff. From first-order levels, October 13 to 16, 1923, by party of 

 E. B. Roberts, the zero of this staff was found to be 17.97 feet below 

 the old bench mark 1 at the navy yard. As the original series of 

 observations at the navy yard was referred to a datum 30 feet below 

 this bench mark, the staff readings of the later series can be made com- 

 parable with the old series by the addition of the constant 12.03 feet. 



Primary bench marlc. — As the tide-staff support in its exposed posi- 

 tion is in more or less danger of destruction or change, it is necessary 

 to check its elevation from time to time by leveling to some bench 

 mark in the near vicinity. The mark adopted as the primary bench 

 mark for this series of observations is designated as "B. M. 7" and 

 consists of a standard disk stamped "7/1922," which is cemented into 

 the upper surface of water table on the side of heating plant opposite 

 Commonwealth Pier No. 5. This bench mark is 21.45 feet above the 

 zero of the tide staff installed in September, 1922. The primary bench 

 mark has also been connected by levels with a number of other 

 bench marks in Boston to further insure the permanency of the tidal 

 ■datums. 



Reduction qf records.- — The records of the observations at Common- 

 wealth Pier No. 5 have been reduced by the usual methods. Monthly 

 means and extremes of the high and low waters are compiled in Tables 

 9 to 24, and summaries of the tidal data are included in Tables 25 to 

 28. Definitions of the various tidal constants have already been given 

 on pages 10-14 in connection with the observations at the navy yard. 



Monthly means. — The monthly means for the first portion of the 

 series of observations at Commonwealth Pier No. 5 to the end of the 

 year 1924 were derived from 29-day groups, beginning with the first 

 of each calendar month. Beginning with the year 1925, the means 

 have been derived from the full calendar months. The record from 

 the first few months of observations was rejected as defective on 

 account of the clogging of the intake to the float well, and the tabu- 

 lated means begin with August, 1921. 



Annual variation. — The several tide planes appear to be subject to 

 certain seasonal fluctuations, as illustrated in Figures 3 to 6, which rep- 

 resent the means from the observations at Commonwealth Pier No. 5 

 covering a period of a little more than five years. By Figure 3 it will 

 be noted that the high water in the summer months is several tenths 

 of a foot higher than in the winter months. Figure 4, which repre- 

 sents the annual variation in the low water, is somewhat similar to that 

 for the high water but shows a secondary depression through the middle 



