HAT 8. 18S5.]
��SCIENCE.
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��tory studies. A second volume is promised to contain the more geogroiiltic topics, wliile Ibe one now issued treats of terrestrial phvaice in & 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 li_v an historical introduction, and fol- lowed by a brief and discriminating discussion of volennoes" and earthquakes ; and aU 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 prejiara- tion of the work, we cannot do better than give in brief abstract an outliDe of three dis- cussions on subjects that have not received sufficient alteuiion on this side of the water, — the irregularities of the earth's shape, the effects of its rotary motion, and the liyjjolhesis that its interior is gaseous.
The development of the belief in the glob- ular form of the earth is treated at length ; and tbe reasons for giving up the Cassininn 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 afler it had to be admitted that meridians measured in different countries could not be fitted on any single elli|> soid, and afler it was fotmd that mountains exerted a sensible lateral attraction on plumb- lines hung at their bases, it was still supfjosed, even by such meu as Gnuss 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, ft'oni the numlwr of oscillations counted in a day at ditferent latitudes. In the course of these dillleult experiments, it was found, strangely enough, that pendulums would swing faster on mld-occanic islands than on the opposite continental coasts : the difl'erencc 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 iliat 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 iuMrrect, i and that the true explanation of the dlfflcnlty ia to be found in the deformation of the ocean's surface by lateral continental atlrnction. Tbe most important investigation of this deformity, and of the many diiHculties it adds to geodetie work, is by Fischer, in a small volume entitled ' Unt«r8uchungen iiher die gestall der erde' (1868). Saigey, 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 ou the coasts, because they are nearer the centre of the
Other important modiQcations of previous views follow from these conclusions ; continen- tal upheaval becomes more of a problem than ever ; the great East-Indian arc is considered useleBB for determining the size and shape of the earth; and Airy's explanation of the ab- sence of lateral attraction by the Himalaya ia pronounced incorrect. Evidently, geodesist* iiave 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 proiwrty 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 Herscbel has explicitly given this meaning. But as a matter of fact, the deflective force is the same, wliatever be the direction of motion from a given point, and the demonstration of this unapparent truth is here simply presented. Still a farther step ia taken in quoting the results of Finger's re- cent investigations, where it is shown that on a spheroid, inst«ad of on a s|)here, it is not precisely true that the deflective force is inde- pendent of the direction of motion : it is great- est for enstwaid motion ; and in conflrmation of this, Giinther quotes Darapsky, who Hnds that in artillery practice the observed deflec- tions are greatest when the aim is directly to the east. The variation is extremely small, and
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