TIDES AND CURRENTS IN BOSTON HARBOR 9 



16-foot mark, which was the lowest division of the scale. It was also 

 determined that the top of the coping was 30.47 feet above the aver- 

 age level of the gate sills of the dry dock. 



Another measurement of this scale, made under the direction of an 

 officer of the Coast and Geodetic Survey on June 27, 1927, indicated 

 an excess varying from 0.04 to 0.08 foot between individual stones and 

 a total excess in the entire scale wliich was 0.04 foot greater than that 

 obtained in 1903. Although this suggests that bench mark 1 may 

 have risen 0.04 foot during the period 1903 to 1927, it will be noted 

 that the levels between bench mark 1 and bench mark 2 show a rela- 

 tive change of only 0.02 foot for this period, and that levels to bench 

 mark 3 indicate a settlement of 0.04 foot between 1916 and 1927. 



Conclusions in regard to iench-marlc changes. — What conclusions can 

 be drawn from the above data in regard to any changes in the elevation 

 of bench mark 1 since the tide observations were commenced in 1847? 

 Changes from three causes are conceivable — settlement of the dock as 

 a whole, raising of the stones by the action of frost in the seams be- 

 tween the stones, and changes caused by relaying the stones during 

 repairs. 



The dock was substantially constructed by a capable engineer; 

 approximately 16 years had elapsed after its construction before the 

 tide observations were commenced; and a careful investigation by 

 the chief engineer of the committee of the Charles Rivor Dam in 1902 

 failed to reveal any evidence of a settlement. For these reasons we 

 may safely disregard the question of settlement. 



Changes due to other causes are in evidence, but the exact amount 

 of such changes is unknown. It will be noted that in 1857 an expan- 

 sion of the seams between stones amounting to approximately 2 

 inches in some cases was reported. In 1903 a careful measurement 

 gave the maximum expansion between two stones as 0.07 foot, or a 

 little less than 1 inch, and in 1927 this expansion was found approxi- 

 mately the same. Taking into account the possibility that the value 

 given in 1857 may have been only a rough estimate rather than a 

 direct measurement, there appears to be no reason for concluding that 

 the elevation of the bench mark on the coping since 1903 has differed 

 materially from its elevation in 1857 because of the opening of the 

 seams between the stones. It appears probable that the expansion 

 due to the action of the frost had already reached a maximum in 

 1857, and it is not impossible that this expansion had reached a 

 maximum prior to that time. As the dry-dock walls had been com- 

 pleted approximately 16 years before the tide observations were 

 commenced in 1847, it is reasonable to assume that there had already 

 been some expansion in the seams prior to the beginning of the obser- 

 vations, and the bench mark may not have changed much in elevation 

 between that time and 1857. 



During the repairs of 1859 some of the stones were relaid, and it is 

 probable that the bench mark on the coping was lowered a little 

 but through the action of frost in subsequent years again raised. 

 Similarly in 1869-70 the bench mark may have been lowered again 

 a small amount by repairs and later raised through the action of frost. 



From the year 1900 to the present time (1927) leveling connections 

 with other bench marks indicate that bench mark 1 has remained 

 very nearly fixed. If all the leveling is accepted as absolutely 

 accurate, bench mark 1 apparently rose 0.02 foot relative to bench 



