596 iiEroRT— 1887. 



10. On the Effect of Continental Lands in altering the Level of the adjoining 

 Oceans. By Professor Edward Hull, LL.D., F.B.S. 



The effect of the attraction of coutiiieutal land upon the oceanic waters adjoin- 

 ing seems to have been very much overlooked by British physical geographers. 

 That some slight effect arises iu the direction of elevating the surface of the ocean 

 in proximity to the coast is generally admitted, but the amount of rise is considered 

 to be small, perhaps insignificant. The prevalence of these views was attributed 

 by the author to the widespread influence of Lyell's hypothesis of the uniformity 

 of the ocean-surface all over the globe. 



The author's attention had been called to the subject by the perusal of the 

 works of the German geographers Suess ' and Fischer/- especially the latter ; and 

 he had received gi-eat assistance in his investigations from Professor G. G. Stokes, 

 Pres. R.S., and from the Rev. Maxwell II. Close, F.G.S., which assistance he 

 gratefully acknowledged. 



In attempting to determine the relative levels of the ocean surface along the 

 margins of continents as compared with those of mid-oceanic islands, the German 

 authors above quoted had based their results on observations of the length of the 

 second's pendulum. Many years ago (1849) Stokes had .shown that the force of 

 gravity must be greater in such islands than on continental stations,^ and this con- 

 clusion corresponded with actual observations on the length of the second's pendu- 

 lum at stations all over the globe as collected by Airy. ' The formula of Suess and 

 Fischer based on these was to the effect that the difference in tbe level of the ocean 

 between two such statinns was found iu ml'trei< by multiplying the difference in the 

 number of daily oscillations in the second's pendulum by 122. This in the case of 

 the stations of California ( or Mexico ?) in lat. 21° 30' and of the Sandwich Islands 

 would amount to 4,520 feet ; a very startling result if correct. 



The author proceeded to discuss the effect of continental lands, showing that 

 this was in the first instance divisible under two principal heads : The effect (1) 

 of the unsubmerged, and (2) of the submerged masses. In the former case, where 

 the mass rose above the surface, one component of the attraction acted in a more 

 or less vertical direction ; in the second case, all in a lateral direction; but both had 

 the effect of elevating the surface of the ocean. The horizontal distance to which 

 the vertical effect extended owing to the curvature of the earth's surface was then 

 considered ; and it was shown that, where continental lands rise from a deep ocean, 

 the effect of the lateral attraction far exceeds that of the vertical attraction of the 

 unsubmerged mass. Professor Stokes had furnished the author with a hypothetical 

 case, in which the elevation of the ocean was estimated to reach 400 feet above 

 the mean geodetic surface of the earth. 



For the purposes of illustration three cases were selected, viz. : — 



(1) The table-land of Mexico, between lats. 18^ and 26° N. 



(2) The table-land of Bolivia, „ 19° and 26° S. 



(3) The Andes of Chile, „ 26° and 35° S. 



The mean elevations, distances from tlie ocean, and extent having been deter- 

 mined, and the mean density being taken at 2'6 for emergent, and I'G for sub- 

 merged land, the results of the attraction of the mountain masses in each case 

 were as follows : — 



(1) Mexico, 2.30 feet ; (2) Bolivia, 301 feet; (3) Chile, 63 feet ; the elevations 

 being calculated above a mean geodetic surface. 



To the above results, due to tlie gravitation-potential of the elevated masses, 

 were to be added those due to the following factors: — 



(«) The marginal plain or emergent tract on either side of the mountain mass. 



(b) The high lands both to the north and south of the special sections 

 above dealt with. 



(c) And lastly, and most important, the submerged continental mass. 



' Suess, Das AntUtz der Erdc (1887). 



' Fischer, Untersvclnmgen riber die Gestalt der E-rde (1886). 



' Stokes, Cambridge Philosophical Transactimts, vol. viii. pp. 672-G95. 



^ Airy, ' On the Figure of the Eartli,' Encyclop. Metrojpolitatia. 



