The Tidal Datum of Hobson's Bay. 147 



used as the datum of all our railway and other systems of 

 levels. No very precise information as to the vertical 

 distance between this bench mark and the tidal datum 

 adopted in the earlier part of the survey of the bay can be 

 found, but Captain Cox, in 1864, after discussion of two 

 years' records of the self-registering tide-gauge, adopted the 

 following : — Sill of lighthouse above high- water springs, 

 7 '27; low- water do., 9*94; mean tide, 8 -60. On a subsequent 

 examination of these numbers it was found that a very 

 obvious error had been made in referring the tide-gauge 

 scales at the tide-gauge house to the lighthouse datum, and 

 that they should have been as follows : — High- water springs, 

 5'95; low- water do., 7'62; mean tide, 6*78. This distance at 

 low-water springs was subsequently adopted by succeeding 

 Admiralty surveyors. 



Nearly all the Admiralty surveyors, as well as others 

 who occasionally made surveys in connection with the 

 silting up of Hobson's Bay, affixed, for facility of refer- 

 ence, a graduated scale to the outside of the tide-gauge 

 house, and dipping the water. These were fixed alongside 

 of and sometimes over one another, and although several 

 have got destroyed by boats, there are still five, differing 

 more or less among themselves. No doubt this proceeding- 

 has been a fruitful source of errors, as well as of discrepancies, 

 in compared results of surveys. 



In 1871 a law case, involving the accurate definition of 

 high- water mark on a part of the shores of Hobson's 

 Bay, led to an investigation of the tide records and a 

 revision of the various tide data. This was undertaken 

 by Mr. A. C. Allan and myself, as already mentioned, 

 and we made a very careful series of measurements 

 of the vertical distance between the sea surface and the 

 various datiims. Thinking it would be convenient, and 

 perhaps facilitate future references, if an imaginary datum 

 below the lowest probable tides were adopted, we reduced 

 all our measurements to a zero 10 ft. below the sill of the 

 lighthouse, and the following will give various points referred 

 to this zero:— 



Feet above datum. 

 Tide Zero ... ... ... 0-00 



Sill of Lighthouse ... ... ... 10 -00 



N.E. corner Dock Basin ... ... 10*55 



Low-water Springs .., ... 2 '38 



B,M. niche Prince's Bridge ... ... 17-69 



Door-step of Observatory ... ... 93-31 



