6 U. S. COAST AND GEODETIC SURVEY 



Repairs to dry dock. — In the years 1869 and 1870 extensive repairs 

 were made to the dry dock, which included the relaying of the coping 

 stones. These repairs necessitated the removal of the tide station to 

 another position in July, 1870. 



Porcelain tide sta/ff. — In the latter part of the year 1869 the tide staff 

 had been reported to be in bad condition. Under date of October 23, 

 1869, A. C. Mitchell was informed that a new porcelain staff would be 

 sent to him for installation, and he was instructed as follows: 



These tide staffs are made of sheet iron, with a porcelain covering on the face 

 side and paint on the back side. They should be handled with some care to avoid 

 bending, lest the covering be cracked. You should select a place for the one at the 

 navy yard where it will be as secure as may be from accidental injury and remain 

 as a permanent scale for reference, and not too far from the tide gauge. It should 

 be fixed up on the stone wall of a wharf, if it can be done conveniently, and where 

 the height of the water can be read on it at any time. We use for economy ordi- 

 nary brass wood screws with thick brass washers between the heads of the screws 

 and the plates. These washers fit into the holes in the plates and protect the 

 coating from injury. The plates can be screwed upon a well-painted pine plank, 

 which must be at least 2 inches thick and a little longer and wider than the plates. 

 The staff is 20 feet long and made in 10-foot sections, and you will find in putting 

 it up that the lower end of the upper section must go a little over or under the 

 upper end of the other piece in order to preserve the proper distance between the 

 divisions. The pine plank that receives the plates may be fixed directly upon the 

 wall if that is perpendicular, but if not, then to another piece of timber properly 

 cut to bring it so, and painted. All the bolts or spikes used should be of brass or 

 copper in order to secure durability. 



If you can do so, it will be best to place the 20-foo-c mark of this staff on a level 

 with the old bench mark at the entrance of the dry dock, which reads 30 feet on 

 its own scale. We believe some of the copper figures and marks are still remaining 

 which will enable you to find that place for the 30. That bench mark has been 

 used a great deal and we know its history, and if you can place the new staff as 

 here directed it will be very convenient in computations, there being just 10 feet 

 difference. We believe the old bench mark has never been submerged more than 

 once or twice when there were very severe easterly storms. 



We suppose you will have to use a level to fix the position of the new staff or to 

 ascertain its height relatively to the bench marks, and while doing so we want 

 you to level between the old bench mark at the entrance to the dry dock and the 

 bench mark fixed by you in August, 1867, near the head of the dry dock. 



In reply to the above Mr. Mitchell wrote under date of November 

 1, 1869, asking for further instructions and stated — 



The old bench mark is again gone, as they have just taken down the lower end 

 of the dry dock. 



In reply to this letter the assistant in charge of the Coast Survey 

 office wrote — 



We infer from your letter of the 1st that the repairs that are being made at 

 the dry dock are so extensive that it is not best for us to put up the porcelain 

 staff until they are completed. When they renew the copper scale af the entrance 

 of the dry dock we shall want to know how it compares with the old one, and it 

 will, we suppose, be necessary for us to do it by leveling. We will therefore 

 defer the whole thing till then. 



On January 12, 1870, A. C. Mitchell wrote — 



Before leaving Boston I found that the marks on the tide staff at the navy 

 yard had become so obUterated that it could not be read at low water. So I 

 ordered John Cass to prepare the pile for the porcelain plates (the same pile 

 being the only suitable place) and instructed him how to put the plates on so 

 that the 18-foot mark would come in the same place as before. I expect that 

 this has been done and also that the dry dock is done. The plates can be moved 

 at any time, so that the 20-foot mark can be on a level with the bench mark, but 

 the sooner it is done the better, before the screws get rusted in. Would it not 

 be best for me to go and attend to that when I get through here? 



