4 U. S. COAST AND GEODETIC SURVEY 



letter from Henry Mitchell, assistant in the Coast Survey, under date 

 of October 21, 1860, in which he writes — 



In a recent letter on tidal matters I gave you information of an error in the 

 position of the Charlestown gauge. I have fully examined this matter now and 

 can make an official report. 



Within the last year the blocks of granite, forming the outer position of the dry 

 dock, have been removed to make repairs; and when these were replaced it was 

 thought advisable to drop them a little below their old positions, as it had been 

 found that the coping of the dock inclined inland. The positions of the copper 

 figures were also altered, so as to make the top of the wall read 30 feet, as before. 



The gauge is placed in a situation which makes it rather inconvenient for level- 

 ing, so that lines run by different persons differ several hundredths. 



We have determined, with the utmost care, that the coping directly over copper 

 figures, instead of reading 30 feet on our gauge, reads 29.93 feet — i.'e., 0.07 lower 

 than before repairs. 



If your mean low water read 14.76 feet below coping before, it should now be 

 14.69 below same point of new waU. 



In my last I stated that the dock coping inclines seaward. I find now that the 

 observer had accidentally reversed his figures and that it still inclines inland 

 over one-tenth foot for the original length of the dock. 



This letter contained a rough sketch showing the position of the 

 tide gauge a short distance to the northeastward of the dry-dock 

 entrance. 



Mr. Mitchell ascertained that the top of the coping which was 

 being used as a bench mark corresponded to a reading of 29.93 feet 

 on the tide gauge instead of 30 feet. It is assumed that this result 

 was obtained by leveling to the tide staff which was located north- 

 easterly from the dry-dock entrance and did not refer to the bronze 

 scale below the bench mark. The 29-foot division of the bronze 

 scale is on the coping stone itself, and no disturbance of the stone 

 would have affected the relation of bench mark to this scale division. 

 Although Mr. Mitchell assumed that the difference of 0.07 foot 

 between 30 feet and 29.93 feet was due to a change in the elevation 

 of the bench mark, it is not at all certain that the tide staff had previ- 

 ously been set with its zero exactly 30 feet below the bench mark. 

 Taking into consideration the wide openings between the stones, as 

 reported by Mr. Williams in 1857, it seems quite possible that in 

 resetting these stones the bench mark on the coping may have been 

 lowered by even a greater amount than 0.07 foot. 



Installation of box gauge. — The tide station appears to have been 

 retained in its position northeast of the dry dock until May 28, 1861, 

 when a box gauge was installed in a tide house a short distance south- 

 westerly from the dry-dock entrance. The following is a copy of 

 Henry Mitchell's descriptions of this gauge: 



A cylindrical copper float 5M by 9 inches (outside) rises and faUs within a 

 copper pipe of 6 inches (inside) diameter, closed at bottom. A wooden box 

 incases the copper pipe and protects the latter, from being jammed by vessels 

 lying alongside the wharf. This box is firmly secured to a pile by iron straps. 

 The water enters the outer box by several half-inch holes near the bottom and 

 enters the copper pipe by six 3^-inch holes, similarly situated. 



The graduated rod, from which the observer reads the height of the tide, 

 passes from a socket in the float upward through the floor of the house built for 

 the self-registering tide gauge. A hole through the roof of the house will allow 

 the gauge rod to pass up to any height. This hole is to be covered^ whenever it 

 storms, and at such times the observer may shut up the hole, it being provided 

 with hinges. Whenever the observer is not at the station the pole is doubled 

 upon its hinges in order that it need not be exposed to accidents by projecting 

 above the roof — the flying-jib booms of vessels frequently project over the gauge 

 house. 



