May 7, 1886.] 



SCIENCE. 



421 



in the density of the crust that this result can be 

 attributed. The only admissible explanation is a 

 diminished distance to the attracting centre, and 

 consequently a deformation of the ellipsoidal sur- 

 face of the sea. 



Hence I express my desire for further measures 

 of great circles, following the suggestions of 

 Bruns, the astronomical and geodetic observations 

 to be combined with the most precise levellings 

 and with measures of the force of gravity. Then 

 only could the question be decided in a definite 

 manner. Up to that time it is premature to wish 

 to attack it, either by hypothesis in discord with 

 the laws of science, or in passing over in silence 

 the work, which, true or false, merits at least a 

 respectful examination. A. de Lapparent. 



II. 



M. de Lapparent's high authority as a geolo- 

 gist renders it my duty to give certain explana- 

 tions in support of the partly geological theory 

 which I have recently presented. 



First, as regards the figure of the earth, it is not 

 a question of authority taken second or third 

 hand. The measurements of arcs of meridi- 

 ans are well known ; and the calculation which 

 permits us to conclude from these measurements 

 the figure of the earth is very simple, and may 

 be verified by any one. 



The surface of the earth conforms so well in 

 all parts with an ellipsoid of revolution, that the 

 deviations are absolutely unappreciable, save by 

 the most delicate measurements. 



As regards the pendulum, with which the most 

 recent measurements have been made by Mr. 

 Clark of England, and in the United States by Mr. 

 Peirce, the results are no less striking. These two 

 reach by the same method of observation, wholly 

 independen of the measurements of arc, and by 

 calculations easily verified, the same flattening, 

 1:292. 



It is very true, as M. de Lapparent has remarked, 

 that, among the numerous observations made in 

 all parts of the earth, those which have been 

 made on the small isolated islands in the middle 

 of the ocean have indicated a force of gravity a 

 little too strong ; but these slight anomalies do 

 not vitiate the general result, that is to say, the 

 value of flattening above given. 



This fact has been known for seventy years, 

 but it has been wrongly interpreted. Some have 

 laid the blame upon the observers. Others have 

 said, that, as the islands were of volcanic origin, 

 the materials composing them have a greater 

 density, which would account for the excess of 

 local attraction. Others, fifty years ago, have 



said, what M. de Lapparent repeats to-day, that, 

 if the force of gravity is a little greater on the 

 islands, it is because the surface of the sea is 

 nearer the centre of the earth. 



The true interpretation is less pretentious, and 

 does not contradict assured scientific facts. It is 

 simply that it has been forgotten to take into ac- 

 count the excess of attraction of the submerged 

 mountains, at the summit of which observations 

 were made, over the attraction of an equal volume 

 of water, which it replaces in the middle of 

 the sea. Unfortunately the navigators have not 

 thought to determine by suitable soundings the 

 form of the submarine pedestal on which their 

 instrument was placed, so that it is impossible 

 to-day to apply the necessary corrections to their 

 results. 



Finally, the chief argument of my opponent 

 is the poor conductibility of the rocks which 

 compose the earth's crust. I will say first, that, 

 despite this feeble conductibility, the earth has be- 

 come sufficiently cool in the course of the geologic 

 ages to have acquired a solid crust of from thirty 

 thousand to forty thousand metres in thickness. 

 It follows, then, that the central heat traverses 

 this thick crust, notwithstanding its slight con- 

 ducting-power, to finally lose itself by radiation 

 in space. I am unable to see that this undoubted 

 cooling operates everywhere under the same con- 

 ditions. Leave aside the argument of Siberia, 

 and consider a spherical surface a league or a 

 league and a half below the surface of the earth. 

 At this depth it is necessary to distinguish two 

 regions, — one situated beneath the continents, 

 and the other found in the depths of the 

 ocean. The central heat which arrives at this 

 surface in the first region must still traverse an 

 enormous bed of rock before it can radiate into 

 space. Precisely on account of the slight con- 

 ductibility of this highly protecting thickness of 

 rock, very little heat passes ; and there beneath 

 our feet, at this depth, the central heat makes 

 itself strongly felt, the temperature rising to more 

 than 200° C. In the other region — the sub- 

 marine region — the case is different. There the 

 superincumbent bed, of a league and a half in 

 thickness, is water ; but water is an excellent 

 transporter of heat when received from the bot- 

 tom, the water carrying the heat upward, not by 

 conduction so much as by the ascending currents, 

 to which the least accession of heat gives rise. 

 Thus the central heat passes easily in such a 

 region. Moreover, the continual flowing-in of 

 polar water at a temperature of — 1° or — 2° aids 

 the refrigeration. 



It therefore seems to me evident that the cooling 

 of the central mass is facilitated by the sea, and 



