SC I E N C E -Supplement. 



FRIDAY, MAY 7, 1886. 



IS THE OCEAN SURFACE DEPRESSED ? 

 I. 



The Revue scientifique published recently the 

 following discussion on the communication made 

 in January at the Sor bonne by H. Faye, upon the 

 permanence of the earth's figure throughout geo- 

 logic times. The eminent academician then af- 

 firmed that accord exists among geodesists as to 

 the figure of our planet ; that the measures of 

 arcs of meridians already made have done away 

 with all irregularities, which at the beginning of 

 this century w r ere supposed to exist ; and that 

 one can assign for the form of the surface of the 

 sea an ellipsoid of revolution, having an eccen- 

 tricity of 1 : 292 (accurate to one unit in the de- 

 nominator). 



I do not feel able to say how the assertions of 

 M. Faye can be reconciled with the diametrically 

 opposite ideas which have been developed hi recent 

 German works, noticeably in the ' Lehrbuch der 

 geophysik' of Gun'ther, the works published in 

 1868 by P. H. Fischer, in 1878 and 1877 by Listing, 

 and, above all, in the important memoir which 

 Brims published at Berlin in 1876 ; which last is 

 not even mentioned by the learned French as- 

 tronomer. I can only call attention to his estima- 

 tion of their value, without being able to judge 

 of the reasons which have determined it. I must 

 leave this to the geodesists. 



I would s^y the same thing of another as- 

 sertion of M. Faye, — that relative to the con- 

 stancy of the force of gravitation at the surface 

 of the sea along the same parallel. " Navigators," 

 says he, " have carried the pendulum at the surface 

 of the sea over a large portion of the earth, and hi 

 both hemispheres, without the pendulum indicat- 

 ing the least diminution of the force of gravity 

 ascribable to depression of the earth's crust." 

 Now, Fischer, as well as Hann, states, that, upon 

 the islands situated in the open ocean, the pendu- 

 lum, when swung at the surface of the sea, exe- 

 cutes at least nine and one-third more oscillations 

 than upon the shores of the large continents. 

 This, at the rate of one hundred and twenty 

 metres for one oscillation, gives more than a 

 thousand metres for the depression of the sea at 

 the centre of the oceans ; and this same conclusion 

 is elaborated also by Listing as well as by Pinck. 

 So startling is this disagreement, that we acknowl- 



edge that it is almost beyond credence ; and. as 

 attention has been called to it, proper experiments 

 should be undertaken to clear away all doubts. 



But, even if we admit the correctness of the 

 data given by M. Faye, there is one point in his 

 theory which we cannot pass over, because it 

 touches the constitution of the earth's crust. The 

 eminent academician reasons somewhat in this 

 way : at any point over the sea, the density of the 

 water being sensibly inferior to that of rocks, 

 there should be a local diminution of the attract- 

 ing mass, and consequently the pendulum ought 

 to oscillate less rapidly. Since this is not the re- 

 sult, there must be some cause counteracting the 

 diminution of the superficial mass. This cause, 

 according to M. Faye, can only be an increase of 

 the density of the crust. As the solid rocks have 

 in general a density greater than that of the 

 molten materials from which they are obtained, 

 and if under the sea the solidification has pro- 

 gressed farther than under the continents, the in- 

 crease of the solid mass under the seas could com- 

 pensate the diminution of density resulting from 

 the column of sea-water above. But to this con- 

 clusion I am not ready to assent. 



If it be true that a majority of bodies are more 

 dense in the solid condition than in the liquid, it 

 is also true that we know very little of the physi- 

 cal condition of the interior of the earth. Even 

 in our day many savants hold that the earth is 

 entirely solid. But, admitting the existence of 

 a liquid interior covered by a solid crust, how can 

 we assert that this crust, traversed by numerous 

 crevasses, does not contain sufficient open spaces 

 to annul the slight increase of density due to 

 solidification. 



Let us accept Faye's hypothesis for the time 

 being, and search with him for the cause w T hich 

 has produced this increase of solidification. We 

 know, from the submarine investigations of the 

 last few years, that everywhere on the bottom of 

 the large oceans there reigns a temperature in the 

 neighborhood of 0° C. The cause of this is to- 

 day well known. The water of the polar regions, 

 rendered denser by cooling, sinks, and, following 

 the bottom of the sea, tends to replace the water 

 evaporated in the tropical regions. M. Faye says 

 this cause for the cooling of the bed of the oceans 

 has existed ever since there have been ice-caps at 

 the poles, and that it is impossible that such an 

 action, prolonged through a sufficiently long 

 period, should not have affected the temperature 



