202 TIDAL AND LEVELLING OBSERVATIONS. 



In 1880-81 the tidal stations at Madras and Bombay were 

 connected for the first time by a line of spirit-levels carried directly 

 across the peninsula, for the most part near the railway. At each 

 station the mean sea-level was determined very approximately, and 

 the result of the spirit-levelling operations over a line 730 miles 

 was to make the mean sea-level apparently about three feet higher 

 at Madras than at Bombay. Compared with the surfnce of the 

 spheroid or other geometrical figure which most closely corresponds 

 to the figure of the earth, there must probably be variations in the 

 general level of the surface of the ocean at different places, and 

 certainly where the attractions of mountains and the like are not 

 counteracted by deficiencies of density in the strata below the 

 elevated masses. But, as the surface of the ocean is everywhere 

 maintained in equilibrium there can be no flow of water from one 

 point to another. The differences of height however considerable 

 must be insensible, because they cannot be measured by instru- 

 mental means, for the causes producing them must equally affect 

 both the spirit-levels of the instruments and the level of the ocean. 

 Had the spirit-levels been carried, without error, along the coast line 

 from Bombay round, via Cape Comorin, to Madras, they must have 

 shown identity of mean sea-level at Bombay and Madras, just as had 

 been met with in the Red Sea and the Mediterranean, on opposite 

 sides of the Isthmus of Suez, and in the Atlantic and the Pacific 

 Ocean at the Isthmus of Panama. And this identity would have 

 been obtained even if there were actually a considerable difference 

 of height, which is very possible ; for the Western Ghats and the 

 generally greater elevation of the western as compared with the 

 eastern half of the peninsula are sources of attraction which if 

 not counteracted by deficiency of density below the elevated masses 

 must raise the mean sea-level at Bombay no less than 31 feet 

 (according to Mr. Hennessey's calculations) above the mean sea-level 

 at Madras. The levels, however, were taken across the continent 

 and not along the coast line ; they were carried from Bombay up 

 the short and abrupt ascent to the crest of the Western Ghats, and 

 then down the long and gentle decline to the east coast, and it has 

 been surmised that the closing discrepancy of three feet at Madras 

 may l^e due to the proximate and local attractions of the hills and 

 table-lands over which the levels were carried, or else to the 

 accumulation of small errors, so minute as to be barely appreciable 

 at any single station, but possessing a tendency to be repeated at 



