S94 



Report of the Great Trigonometrical Survey. [Nov 4 



be re-levelled, on account of large discrepancies which were found in 

 the Bailway levels. The operations had reached the vicinity of Bha- 



one, the greater rise in the river can clearly be attributed only to the narrow- 

 ness of its channel compared with the bay ; it is probable that a considerable 

 portion of the rise of ten inches in the surface of the lake is also due to ac- 

 cumulation j so that, although a rise may be supposed to take place in the level 

 of the sea at the head of the bay, during the continued pressure of the S. W tf 

 Monsoon, still, that elevation must be much less than what takes place in the 

 lake, where the effect of this rise is increased by the narrowness of the channel 

 and the influx of fresh water during the inundation. 



" It has been shown that if the annual average of mean water be taken as the 

 sea level, it would lead to the inadmissible conclusion that, in the dry season, 

 the average level of the river at Calcutta, is twenty-nine inches below the sea f 

 with which it freely communicates. It has also been shewn that the surface of 

 the Great Salt Water Lake, in the dry season, is on a level, or nearly so, with 

 the mean tide of the river at the same time. It is likewise manifest that the 

 periodic rise of mean tide during the monsoon, to the extent of six feet in the 

 river and ten inches in the lake is occasioned by local causes, independent 

 altogether of the true level of the sea, which is a constant level, and these 

 causes, it appears, operating in narrow channels, are capable of producing exag- 

 gerated results in the proportion of seven to one, showing clearly the fact of 

 accumulation. Hence the conclusion is inevitable, that the lowest monthly mean 

 tide of the river, observed in February and March, represents the nearest 

 approximation to the actual sea level, and that the rise of mean tide at Calcutta 

 during other months, may fairly be ascribed to disturbing causes of an inland 

 character, altogether independent of the true and constant level of the ocean. 

 The variable character of the disturbing causes is shewn by the fact that the 

 monthly means of corresponding months for the two years differ considerably, 

 except in the months of February and March, the monthly mean tides of which 

 are very accordant. 



" Proceeding upon this principle, I have used the following observations to 

 refer the datum line in Kydd's Dock to the sea level : — 

 «*Mean Tide February, 1847, above datum, as measured on Guage, ., 

 „ March, „ „ „ „ 



„ February, 1848, „ „ „ 



„ March, „ „ „ n 



„ February, 1850, „ „ „ 



„ March, „ „ „ „ 



„■ February, 1851, „ „ „ 



,j March, „ J? 99 n 



Mean, ... 8'343feet, 

 M Correction for Error of Graduation on Guage by Mr. Bedford's 



Measurements, ... ... ... ... ... 0'233feet, 



* By Tides measured at Calcutta in February and March* Mean Sea 



Level above datum, ... ... ... ... 8*576 feet. 



" Again, in the years 1850 and 1851, Mr. Bedford, the Marine Surveyor, took 

 a series of tidal observations at Kejiri, and connecting this point by a series of 

 levels with Kydd's Dock, found that the datum line at the latter point is 9*07 

 feet below the sea level. Mr. Bedford's observations from which this result is- 

 derived, are as follows : — 



feet, inches. 

 " Mean Height of Sea Level above the datum line at Kejiri, ... 8 975 



"Datum Line at Kejiri above that of Kydd's Guage, 2*88 



f( Sea Level above the datum line of Kydd's Guage, 9 0"63 



811 feet 



8-45 



32 



8*48 



)> 



8'50 



n 



8-28 



if 



8-62 



ft 



7'94 



rj 



8'36 



)> 



^ Which reduced to decimals of a foot becomes^ 



9-053 



