318 F. Guthrie, LL.B. — Sea Level in South [Feb. 27o - 



SEA LEVELS IN; SOUTH AFRICA FROM BAROMETRIC 



OBSERVATIONS. 



By F. Guthrie, LL.B. 



[Read 27th Febeuaey, 1889.1 



The table for finding sea level altitudes from barometric observations 

 which is this evening submitted to the Philosophical Society, though 



calculated according to the ordinary formula ^ log — i, is nevertheless 



h2 



empirical because the coefficient ft has not been determined according 



to theory, but has been assumed, as being 64,300, so as to give correct 



results in certain cases where these results are capable of independent 



verification. 



The observations on which the table is based are those which have 

 been taken at the Royal Observatory and at Kimberley. Other points 

 where sufficient observations have been taken and where the results can 

 be checked by railway levelling must be regarded as points of verifi- 

 cation. Of these the principal are Aliwal North, Cradock, Graham's - 

 Town and Port Elizabeth. As the levels calculated by this table and^ 

 those ascertained by telescopic spirit-levelling agree, within a few feet,, 

 the result must be regarded as satisfactory. 



At present however the number of such points of verification is too 

 small for thoroughly testing the table, which must therefore for the 

 present be regarded as having only provisional value. 



The table is not intended to be used to ascertain sea level from a 

 few barometric observations, it can only be used when a continuous 

 series of daily observations have been taken at a fixed hour. 



When such observations have been taken for four or five years at 

 any place, the sea level of that place ought to be deduced from the 

 table with a probable error of less than 5 feet. 



If only one year's continuous observations have been taken, the 

 average for that year should be corrected for the average deviation 

 that year from the general average over the Colony, and the result 

 should still be correct within some 10 feet or so. 



