Mat S, 1885.] 



SCIENCE. 



387 



tory studies. A second volume is promised to 

 contain the more geographic topics, while the 

 one now issued treats of terrestrial physics in 

 a more general sense under such headings as 

 the relations of the earth to the other planets, 

 the form of the earth, the effect of its motion, 

 and the condition of its interior. These are 

 preceded b} r an historical introduction, and fol- 

 lowed by a brief and discriminating discussion 

 of volcanoes and earthquakes ; and all the 

 chapters are closed by extended lists of cita- 

 tions that add greatly to their value. As 

 indicative of the careful and learned investi- 

 gation that has been required in the prepara- 

 tion of the work, we cannot do better than 

 give in brief abstract an outline of three dis- 

 cussions on subjects that have not received 

 sufficient attention on this side of the water, — 

 the irregularities of the earth's shape, the 

 effects of its rotary motion, and the lrypothesis 

 that its interior is gaseous. 



The development of the belief in the glob- 

 ular form of the earth is treated at length ; and 

 the reasons for giving up the Cassinian view 

 of its elongated polar diameter and accepting 

 the Newtonian explanation of its polar flatten- 

 ing are clearly stated before mention is made 

 of the difficulties that have been encountered 

 in attempting to reconcile the accurate arc- 

 measurements of modern times with the sup- 

 position that the earth must have a regular 

 form. It is then shown, that after it had to be 

 admitted that meridians measured in different 

 countries could not be fitted on any single ellip- 

 soid, and after it was found that mountains 

 exerted a sensible lateral attraction on plumb- 

 lines hung at their bases, it was still supposed, 

 even by such men as Gauss and Bessel, that 

 the ocean was essentially level, and that it 

 would serve as a proper fundamental surface 

 to which measurements of altitude, or distance 

 from the earth's centre, could be referred. 

 During the prevalence of this opinion, through 

 the first third of this century, careful observa- 

 tions of swinging pendulums were made in 

 many parts of the world ; for, as the pendulum 

 moves in obedience to gravity, the flattening 

 of the earth could be deduced, it was thought, 

 from the number of oscillations counted in 

 a day at different latitudes. In the course 

 of these difficult experiments, it was found, 

 strangely enough, that pendulums would swing 

 faster on mid-oceanic islands than on the 

 opposite continental coasts : the difference was 

 small, only eight or nine seconds a day; but 

 it was persistent, and, as it implied a greater 

 strength of gravity, it soon led to the conclu- 

 sion that the earth was denser beneath the 



oceans than below the continents. This view 

 is now widely quoted, and it probably will 

 long remain in our text-books ; although there 

 can be little doubt that it is quite incorrect, 

 and that the true explanation of the difficulty 

 is to be found in the deformation of the ocean's 

 surface by lateral continental attraction. The 

 most important investigation of this deformity, 

 and of the many difficulties it adds to geodetic 

 work, is by Fischer, in a small volume entitled 

 4 Untersuchungen iiber die gestalt der erde ' 

 (1868). Saige} 7 , Stokes, and Hann have also 

 considered the question ; and, although it is not 

 yet possible to say how much the sea is drawn 

 up on the flanks of the continental masses, it 

 is sufficiently demonstrated that the lifting 

 amounts to many hundred feet on certain 

 coasts. As a result, islands appear in mid- 

 ocean that would be submerged if the ocean's 

 surface were really level ; and pendulums must 

 naturally swing faster there than on the coasts, 

 because they are nearer the centre of the 

 earth. ' 



Other important modifications of previous 

 views follow from these conclusions ; continen- 

 tal upheaval becomes more of a problem than 

 ever ; the great East-Indian arc is considered 

 useless for determining the size and shape of 

 the earth ; and Airy's explanation of the ab- 

 sence of lateral attraction by the Himalaya is 

 pronounced incorrect. Evidently, geoclesists 

 have still much to do. 



Among the consequences of the earth's 

 rotation, Giinther gives a full and precise ac- 

 count of the lateral deflection of horizontal 

 motions so conspicuously seen in the oblique 

 motion of the trade-winds. There is not to be 

 found an English text-book on physical geogra- 

 phy in which this matter is properly explained : 

 when mentioned, it is almost invariably stated 

 that the deflective force acts only on north or 

 south motions, and is nothing on bodies mov- 

 ing east or west. Even Herschel has explicitly 

 given this meaning. But as a matter of fact, 

 the deflective force is the same, whatever be 

 the direction of motion from a given point, 

 and the demonstration of this unapparent truth 

 is here simply presented. Still a farther step 

 is taken in quoting the results of Finger's re- 

 cent investigations, where it is shown that on 

 a spheroid, instead of on a sphere, it is not 

 precisely true that the deflective force is inde- 

 pendent of the direction of motion : it is great- 

 est for eastward motion ; and in confirmation 

 of this, Giinther quotes Darapsky, who finds 

 that in artillery practice the observed deflec- 

 tions are greatest when the aim is directly to 

 the east. The variation is extremely small, and 



