1864.] Report of the Great Trigonometrical Survey. 



393 



ed to re-level the line, as soon as the Engineers had leisure to do so. 

 I decided, therefore, on deputing the Levelling Party to re-level the 

 line of the Eailway, and connect all the Trigonometrical Stations 

 within reach thereof. 



Mr. Donnelly made good progress, and accomplished two hundred 

 and forty-two miles of first-class levelling, 1 * forty-one of which had to 



# With an Assistant levelling the line, independently, behind him, station by 

 station, after the method described in the published volume of Tables of 

 Heights. 



The following description of the connection of Kydd's Dock with the mean 

 sea level of the Bay of Bengal is taken from a Report, dated 1st November, 1854, 

 on the Calcutta Meridional Series, by Colonel Waugh, Surveyor- General, and 

 Superintendent Gr. T. S. : — 



" A Register of the Tides in the River Hoogly is regularly kept at Kydd's 

 Dockyard, near Calcutta, the height of each successive tide being referred to a 

 fixed datum line or zero, which is the bottom or sill stone of the dock, and 

 therefore, an object of invariable character. 



" A transcript of the Register of the Tides for two years viz., — from May, 

 1846, to April, 1848, having been obtained from the Marine Department, a 

 Monthly Abstract of Mean Tides was deduced therefrom. 



" The waters of the ocean would maintain a constant level if undisturbed 

 by the action of the Sun and Moon. La Place has demonstrated that this level 

 is a mean between the highest and lowest state to which the surface of the 

 ocean is reduced by the attraction of those bodies. This mathematical truth is 

 corroborated by observations made on open coasts, from which it results that 

 the mean of high and low water for two consecutive tides represents, very 

 nearly, the level of the sea, and that the average for a lunation is constant 

 within a very small quantity. — Vide Professor Whewell's Report, 7 vol., 

 British Association's Report 



" An examination of the Abstract of Monthly Mean Tides will, however, 

 show that considerable irregularity exists in the River Hoogly, the monthly 

 means differing as much as six and a-half feet. Now, if the annual average be 

 considered as the true level of the sea, it would follow that for some months, 

 consecutively, the mean height of the River is two and a-half feet below the 

 sea level, a conclusion which is altogether inadmissible. 



" The lowest monthly mean tide occurs about February and March, when the 

 fresh water in the river is lowest, and strong Southerly winds do not prevail. 

 The mean tide rises gradually, as the river rises during the South Monsoon 

 until it attains its maximum in September or October, at which time the 

 monthly mean exceeds that of February by no less than six feet. This rise is, 

 obviously, the effect of accumulation, produced by inundation in the valley of 

 the Ganges, and the force of the South- West wind, which dams up the freshes 

 in the long and narrow channel of the river. 



" It has been remarked by Colonel Cheape, Chief Engineer, in his Memoirs, 

 dated April, 1825, that the surface of the Salt Water Lake, wherein the rise of 

 the tide is almost imperceptible, would, on account of its wide expanse, repre- 

 sent very accurately the level of the sea with which it communicates. He also 

 observes that Captain Taylor's levels indicate that the surface of the lake in 

 the dry season, is 2/. 4*3ms. below the mean state of the river. This result 

 corresponds very nearly with the mean tide of the river itself, which in 

 February is 2/. 5ms. below the level of the annual mean. 



" Colonel Cheape further states that the periodic rise of the surface of the 

 lake in the wet season is ten inches. Now, the contemporaneous rise in the 

 mean tide of the river has been shown to be six feet, and as the cause of these 

 elevations is precisely the same, though the effects are in the ratio of seven to 



3 B 



