Hull — On the Effect of Continental Land on Level of Ocean. 653 



four hundred feet. Of course in a great continent it might be con- 

 siderably greater. This would cause an apparent diminution of gravity 

 in continental stations — I mean, of course, in gravity as reduced by 

 the usual methods to the level of the sea. In the first place, in re- 

 ducing to the level of the sea we leave out of consideration the attrac- 

 tion of the stratum of earth between the actual sea-level and what the 

 sea-level would have been if the continent had been away. As far as 

 this goes, corrected gravity ought to appear too great. But in the 

 second place, in reducing to the level of the sea, we reduce to a point 

 further from the centre of the earth than we should have done if the 

 sea-level had been unchanged ; and therefore in correcting we don't add 

 enough to bring it up to what it would have been if ocean had been 

 beneath us instead of land. On this account, therefore, gravity should 

 appear too small — I mean reduced gravity. The two effects on ap- 

 parent gravity are antagonistic, but the second is the stronger, so that 

 on the whole gravity ought, cateris paribus, to appear a little less on 

 continents than on detached islands. Sabine and Airy have pointed 

 out that such appears to be the result of observation ; but so far as I 

 know, I was the first to point out that such a result ought to follow 

 from the attraction of a continent by disturbing the sea-level. 



' ' Far inland the thing could only be tested in those cases where 

 the sea-level has been accurately determined by geodetic operations. 

 We could not accordingly throw much light on the question by means 

 of pendulum observations in Thibet. 



" I am, dear sir, 



"Yours very truly, 



" G. G. Stokes. 

 "E.Hull, Esq., F.E. S. 



"P.S. — You will find Airy's discussion in his article, 'Figure of 

 the Earth,' in the Encyclopedia Metropolitana. 



" An elevation of four hundred feet, even if there had been no in- 

 tervening attraction to reduce the resulting diminution of gravity 

 would only alter the number of vibrations per diem of a seconds pen- 

 dulum by about one and a-half. Of course, apart from disturbance, the 

 difference in the number per diem at two stations on the same parallel 

 of latitude would be nil, and therefore the small difference of 1*5 would 

 be infinity times that. I do not know what the term of comparison 

 used by Fischer may be." 



SCIEN. PKOC. K.D.S. — VOL. V. FT. VIII. 2 Z 



