174 HISTORY OF GEOLOGY AND "PALEONTOLOGY. 



methods which were then known ; it is an imago mnndi, or 

 mirror of the world, of the most faithful kind. 



Immediately before the publication of Humboldt's Cosmos, 

 in 1844, Bernhardt Studer, the Swiss geologist, published a text- 

 book of physical geography and geology, which is remarkable 

 for its clearness of disposition, mastery of the subject, familiarity 

 with the literature, and conciseness of treatment. 



Numerous text-books of physiographical geology appeared 

 in the latter half of the nineteenth century; amongst others 

 may be mentioned those of Oscar Peschel (1879), f Siegmund 

 Giinther (new ed., 1897-99), tne popular La Terre of Elisee 

 Reclus (1868-69), those of Hann, Bruckner, and Kirchhoff, 

 and the able chapters in Sir Archibald Geikie's Text-book of 

 Geology (3rd ed., 1893). 



Form, Size, and Weight of the Earth. The determination 

 of the form, the size, and the weight of the earth, although of 

 great interest to geologists, is more especially the domain of 

 the geographer, and cannot here in the narrow limits of space 

 be treated with historical detail. Suffice it to state the present 

 standpoint of our knowledge. For the actual form of the 

 earth, with its numerous deviations from the spheroid of 

 rotation, Listing proposed in 1872 the name of "Geoid," and 

 it is at present one of the chief tasks of the International Com- 

 mission for the measurement of the degree to arrive at the true 

 form of the geoid. 



The form of the geoid, however, cannot be discovered 

 merely by trigonometric methods ; probably the pendulum will 

 play an important part in the future solution of the problem. 

 It has already been demonstrated that the oscillations of the 

 pendulum do not everywhere depend upon the distance from 

 the earth's centre; it is more especially in the interior of con- 

 tinents that the deviations indicate a diminution in the force 

 of gravity. Faye is therefore of opinion, that in consequence 

 of the stronger cooling, the earth's crust is denser under the 

 floor of the ocean than under the continents. Helmert, 

 Hergesell, Drygalski, and others, have supported Faye's hypo- 

 thesis in its main features ; they are of opinion, however, that 

 the attractive force exerted by continents on neighbouring 

 ocean surfaces is more or less compensated for by the smaller 

 density of the earth's crust under the continents. 



The pendulum observations made by Von Sterneck in the 



