Z U. S. COAST AND GEODETIC SURVEY 



also contain summaries of the principal tidal and current elements 

 for several thousand places throughout the world. Other publica- 

 tions of the Coast and Geodetic Survey relating to tides and currents 

 are listed in the back of this volume. 



Changes in the physiographic features of Boston Harbor during the 

 past 150 years are illustrated by a comparison of a map published in 

 England in 1775 (fig. 1) with a modern chart of the Coast and Geodetic 

 Survey covering this harbor (fig. 2). 



TIDE OBSERVATIONS, BOSTON NAVY YARD 



The first primary tide station in Boston Harbor was established at 

 the Charlestown Navy Yard , June 1 , 1847, under the direction of Lieut. 

 C. H. Davis. In the preceding year there was a short series of observa- 

 tions covering the period October 30 to November 20, this short series 

 constituting the earliest tide observations in Boston Harbor of which 

 we have record. 



Original tide scale. — At first the observations were made by reading 

 the height of the water on a tide scale located at the old dry dock. 

 These readings were generally taken every five minutes near the times 

 of high and low water, beginning about a half hour before the esti- 

 mated time and continuing until after the tide had turned. The tide 

 scale was located on the southern side of entrance to the old dry dock 

 (Dry Dock No. 1), just outside the gate. The scale consisted of copper 

 or bronze strips inlaid in the stone wall of the dry dock, these strips 

 having presumably been set in place during the construction of the 

 dock some 16 years before the tide observations were commenced. 



In addition to the scale used for the tide observations, there were 

 five other similar scales in the walls of the dry dock, four of these being 

 inside the dock and the other, like the tide scale, outside the gate. 

 Apparently, the inlaid strips of each scale had been originally spaced 

 at 1-foot intervals, although during the period of observations and 

 afterwards it was reported that the opening of the seams between the 

 stones of the wall had caused the distance between strips on adjacent 

 stones to be slightly in excess of 1 foot. Each even foot of the scales 

 was marked by a numeral, also inlaid in the stone. The scales inside 

 the dry dock were complete, with the zeros corresponding with the 

 level of the dock sills and the 30-foot elevation corresponding with the 

 top of the coping, the last inlaid strip being 1 foot below the coping at 

 the 29-foot elevation. The two scales outside the gate, including the 

 tide scale, commenced with the 16-foot mark and ended with the 29- 

 foot mark just 1 foot below the top of the coping. 



Auxiliary staff.— It was reported that an auxilary staff was used for 

 the tide observations to obtain the subdivisions of the foot, and such 

 a staff was presumably used also to obtain readings below the 16- 

 foot mark of the scale. Just how this auxihary staft' was set is not 

 known. Although it was reported from time to time that the zero of 

 gauge was 30 feet below the top of the coping above the scale, which 

 had been adopted as a bench mark, it is quite reasonable to assume 

 that in the actual setting any one of the footmarks of the auxiliary 

 staff may have been brought opposite the corresponding footmark of 

 the inlaid scale. In such case the distance of the staff zero below the 

 bench mark would have exceeded 30 feet by an amount equal to the 

 excess expansion of the seams between the stones lying above the 

 graduation used in setting the auxiliary staft". 



