TIDAL AND LEVELLING OBSERVATIONS. 203 



successive stations, and to attain some magnitude at the end of a 

 long line. 



During 1881-82 large discrepancies were for the first time met 

 with between the tidal predictions and the actual facts of the tides. 

 This arose in the case of stations situated on the banks of great 

 rivers, as the Hugli and the Irawadi, in which the tides are 

 influenced not only by the attractions of the sun and moon, but by 

 the amount of water brought down by the river from its sources, 

 which varies at different seasons of the year. The tide tables 

 for three stations on the Hugii, two on the Irawadi, and one 

 on the Salwen were found to be erroneous to the extent of 

 occasionally an hour or more in the times and a foot or more in 

 the heights of high and low water, facts which have shown 

 the necessity for supplementing the mathematical formulae for the 

 harmonic analysis of purely luni-solar tides by formula? to take 

 cognizance of riverain influences. The subject is, however, a very 

 abstruse one, and in the meantime Mr. Roberts has adopted a 

 provisional method of predicting tides for riverain ports.* 



One of the results of the Indian tidal observations proved about 

 this time to furnish some light on the question of the rigidity of 

 the earth. The subject had been mooted about 1865 by Sir William 

 Thomson, who appealed to the universal existence of oceanic tides 

 of considerable height as a proof that the earth as a whole possesses 

 a high degree of rigidity, and maintained that the jDreviously 

 received geological hypotheses of a fluid interior were untenable. 

 At the Southampton meeting of the British Association for the 

 Advancement of Science Professor G. H. Darwin brought forward 

 a numerical estimate of the rigidity of the earth, which gives 

 evidence of " a tidal yielding " of the earth's mass, and further 

 indicates that the effective rigidity of the whole earth is about equal 

 to that of steel.f This theory he largely based on the results 

 derived from the Indian tidal observations.! 



An interesting fact was also revealed from a comparison of the 

 observations at Madras with those taken in 1821 by Colonel De 

 Haviland, of the Madras Engineers, which is that the mean sea-level 



* See Tide Tables for 1883 for Diamond harbour in the River Hugli. 



| Professor Darwin's later researches tended to prove that the effective rigidity is 

 not so great as that of steel. 



% See also the address of General J. T. Walker, as President of the Geographical 

 Section of the British Association at the Aberdeen meeting in 1884. 



