246 



SCIENTIFIC NKWS. 



[Mar. 16, I S 



disc by its rapid rotation in the magnetic field produced 

 by the magnet, becomes the seat of induced currents, 

 which are the more intense the greater the speed 

 of rotation. There is then a mechanical action 

 between the disc, which is the seat of the induced 

 currents, and the magnet which produces the magnetic 

 field. The equilibrium of the disc can exist only if the 

 magnetic attraction is precisely equal to the repulsion pro- 

 duced by the magnet upon the induced currents in the 

 disc. As this repulsion preponderates as long as the disc 

 has a sufficient speed, nothing is more natural than this 

 fact — paradoxical as it is in appearance — 0/ a disco/ soft 

 iron repelled by a magnet. When the velocity becomes 

 insufRcient, the attraction preponderates, and the disc is 

 again attracted. 



If the magnet be presented, not to one of the surfaces 

 of the disc, but at the edge in the plane of the disc, no 

 repulsion ensues, and we have then attraction just as if 

 it were at rest. This experiment confirms the explana- 

 tion just given, for in this case no induction currents 

 can be produced, since the disc moves in the plane of the 

 lines of force generated by the magnet. 



This induction top is a simple and useful apparatus 

 for demonstration, and ought to find a place among the 

 appliances for elementary instruction. 



— ■»-^>»^i>^*<f-» — 



ZOOGEOGRAPHY. 



'ZOOGEOGRAPHY is a branch of science of recent 

 •^ times, being one of those numerous and important 

 stems which under the fostering influences of modern 

 research have budded forth from the bare original parent 

 trunk. Natural History. 



Zoogeography is the name given to that science which 

 deals with the geographical distribution of animals. It 

 aims not only at enumerating and classifying the animals 

 of the world, and observing their various ranges and 

 habitats, but after duly noting the facts so observed 

 there arise more difficult questions to which it seeks to 

 find an answer. Why are certain animals found in one 

 part of the world and not in another apparently equally 

 well suited for this existence ? Why is the range[of one 

 species, genus, family, or order extremely limited, while 

 another may extend over large tracts ? Why does the 

 same genus appear in two widely separate localities 

 while unknown in the intervening space ? These are 

 some of the problems which it is the work of the 

 zoogeographer to find a solution. 



For convenience in working the land surface of the 

 globe has been divided into zoological regions corre- 

 sponding to different more or less well-defined typical 

 groups of animal inhabitants. The division established 

 by Mr. Sclater, in 1S37, is the one most generally 

 received, and others differ from it only in comparatively 

 small details. Sclater adapts six regions known as the 

 Palaearctic, Nearctic, Neotropical, Ethiopian, Oriental, 

 and Australian. Each of these regions has its prominent 

 features, but none more so than the Australian, which 

 as far as the mammalia are concerned, is tenanted 

 almost exclusively by the Order IVIarsupiala. Not only 

 is this so, but,, with a single exception — the American 

 Opossum — the Marsupialia are found nowhere else. To 

 explain these and similar phenomena we must call 

 geology and evolution to our aid. 



In the case of the Australian fauna, a study of fossil 

 remains teaches us that the first appearance of mammals 



practically occurred with the commencement of the 

 tertiary epoch. The earliest beds of this epoch are those 

 of the Eocene formation, and we find that the only order 

 of mammals then existing was that of the Marsupialia, 

 which were not then confined to Australia but spread 

 over the world. The higher orders did not appear till 

 late, though where they have arisen through the process 

 of development the marsupials from which they sprung 

 have disappeared. Thus Australia at the present day 

 presents a vast remnant of the old universal Eocene 

 fauna, while the rest of the world has completely dis- 

 tanced it in the race of development. This stagnation 

 on the part of Australia alone is explained by the com- 

 parative smalless of its area, and the very great antiquity 

 of the sea channel, which from the days of the Eocene 

 fauna has cut it off from the rest of the world. 



The solitary existence of the opussum in America, and 

 similar cases where species or families are widely 

 separated, also find their explanation from the facts of 

 evolution. Submerged continents have by some been 

 supposed to account for such phenomena, but evidence 

 is opposed to such an explanation. Much light is thrown 

 upon zoogeography by the study of Island Fauna. Two 

 kinds of islands are recognised in phj'sical geography — 

 the Continental and the Oceanic. 



Continentalislands are those which have been formeily 

 connected with adjacent continents, and on these, as 

 might be expected, the fauna corresponds with that of 

 the mainland from which they have been severed ; and 

 where the separation is one of long standing the animals 

 present proof of modification from the original type. 

 Examples of recent islands are Great Britain and Japan. 

 Of ancient islands, Madagascar and New Zealand are the 

 best type. 



Oceanic islands are those which have come into 

 existence as islands, and which from their origin have 

 remained unconnected with any land. Such are volcanic 

 islands, and coral islands ; as, for example, the Azores, 

 Tahiti, Galapagao, etc. To explain in a natural manner 

 how these islands have become clothed and populated 

 with plants and animals would at first sight appear 

 difficult, but the observations of Darwin and others prove 

 in what a wonderful variety of wa3fs seeds, eggs, plants, 

 and animals may occasionally be transported across 

 immense tracts of ocean. 



Among the most important of these may be mentioned 

 storm-winds and ocean currents. 



Many questions connected with zoogeography still 

 remain to be solved, but of the great importance of the 

 subject there can be no doubt, and it is probable that the 

 full value of its teaching can only be known as years roll 

 on. Of this we have the high evidence of Darwin 

 himself, who talks of "that grand subject, that almost 

 keystone of the laws of creation — geographical dis- 

 tribution." 



The British and French National Libraries. — The 

 statistics of the Bibliotheque Nationale of Paris show that 

 71,932 readers used that institution during the course of 

 a year, and borrowed 213,744 works, or an average of three 

 books to each reader. In the British Museum during 1884 

 there were 1 54,729 visitors to the reading-room, who borrowed 

 1,100,450 works; in 18S6 the numbers were 176,893 and 

 1,247,888 respectively, giving in each case an average of 

 between seven and eight books per reader. Thus, while 

 the readers in the British institution are more than twice as 

 numerous as in the French, each of the former borrows from 

 two to three times as many books as the French reader. 



