Art. VII. — The Tidal Datum of Hobson's Bay. 

 By R. L. J. Ellery, F.R.S. 



[Read 14th August, 1879.] 



The chief object of these brief notes is to place on record 

 in the transactions of our Society as succinct a history as I 

 am able to gather of the tide or sea-level datum of Hobson's 

 Bay. The history of measurements hitherto made is some- 

 what broken, and when, on several occasions, questions have 

 arisen regarding high-water mark, mean tide level, &c, 

 search had to be made in various directions, and the results 

 obtained were found to be doubtful and discrepant. As it 

 is of the highest importance in all matters concerning the 

 conservation of our port, as well as in the disposal of littoral 

 lands, that we should possess trustworthy and precise data, 

 I propose, after sketching a brief history of the question, to 

 furnish the height of various bench marks and points with 

 reference to accurate measurements made a few years ago by 

 Mr. A. C. Allan and myself, with the hope that the vagueness 

 that has hitherto surrounded this question may be set at 

 rest. 



A self-registering tide-gauge was first erected in the 

 waters of Port Phillip at Williamstown in 1858, by the 

 Harbour Department, and placed in charge of the Obser- 

 vatory in June of that year. Soon after, however, it was 

 transferred to the care of the Admiralty surveyors, and it 

 was not until January, 1874, that it came again under the 

 supervision of the Observatory. Since that time a careful 

 and continuous record of the tides has been secured and 

 tabulated every week. Prior to the erection of the gauge 

 the tidal datum was obtained by regular eye observations 

 of the surface of the water at the mouth of the river, 

 and at one or two other places in Hobson's Bay. These 

 observations, however, were taken principally for the 

 purpose of indicating to the shipping the height of water in 

 the river, and furnished no accurate data as to sea level. 



In the earlier operations of the marine survey by Captain 

 Ross, a datum was established by means of a graduated tide 

 board, referred to the bench mark on the sill of the light- 

 house, Gellibrand's Point, which was, and is, I believe, still 



