Dec, 26, 1878] 



NATURE 



^11 



same time the number of persons remaining in the crowd will 

 represent the quantity of heat in, and so the temperature of, 

 the interior. Then it will be obvious that as the crowd disperses 

 the number of persons at any one time between the lines may 

 continue about the same (although the individuals will be 

 changed), whilst those in the central crowd become fewer and 

 fewer. This illustrates how the temperature of the crust may 

 continue nearly uniform in spite of the continued loss of heat 

 from, and cooling of, the interior. 



I believe that I have long ago proved that the mere cooling of 

 a solid earth would not give the amount of contraction needed to 

 account for the observed inequalities of the surface, and I sur- 

 mise that a diminution of the earth's volume has been caused by 

 the escape of steam and gases from volcanic vents during past 

 ages. This view has, however, attracted more attention in 

 America than at home. O. Fisher 



Harlton, Cambridge, December 13 



Magnetic Storm, May 14, 1878 



I AM inclined to think that Mr. Mance's observations (vol. xix. 



. 148) upon the earth currents obser%ed at Kurrachee must be 

 Incorrectly reported. To agree with the obser\'ations in China, 

 Stonyhurst, Greenwich, and Haverfordwest, they should have 

 commenced at 4 a.m. on May 15, and terminated at 5 P.M. on 

 the same day (Kurrachee time). 



It is a pity that electricians do not record these currents in 

 absolute units. To say that the current was equal to fourteen 

 Daniell cells means nothing unless the resistances present are 

 also given. If an earth current is observed upon a cable it is 

 easy to reproduce this current upon the same galvanometer with 

 a known resistance and a known electromotive force, and then 

 to exparess its value in webers or milliwebers. Thus if at Kur- 

 Tachee 50" were noted on a galvanometer, and one Daniell cell 

 reproduced this deflection through a total resistance of 125 



ohms, then the current would be equal to -i- , or "008 weber 



or 8 milliwebers, a magnitude which every electrician would 

 understand. Moreover, if the length, resistance, and general 

 ■direction of the cable or wire were given, as well as the direction 

 of the current itself, the difference of potential of the earth at 

 the two ends would be known. This if the cable were 246 

 •miles long, and lay due east and west, and its resistance were 

 3« per mile, then in "the above case 



— = -008 

 1230 



E = 9-84 volts, 



■which is the difference of potential of the two ends. 



If simultaneous observations were made in this way at nume- 

 rous stations on the earta's surface, we should be able to plot out 

 the distribution of potential on the globe, and arrive at seme 

 betto" knowledge of the cause of earth-currents than we have at 

 present. W. H. Preece 



December 20 



The Derivation of Life from the North 



Attention- has been called by the President of the Royal 

 Society to the labours of Mr. Dyer, as pointing in the case of 

 plants to the conclusion that their various forms have been 

 developed and dispersed from the north, I presume it Is recog- 

 nised that similar conclusions have been arrived at by Mr. A. R. 

 Wallace in the case of animals, Mr. Wallace points to the 

 pabsarctic region as the great centre of their development or crea- 

 tion. On reading " The Geographical Distribution of Animals " 

 when it first appeared, I was so much struck with the evidence 

 adduced, that I was tempted to write and ask him if his work might 

 not be said to occupy the following position in the history of 

 unravellmg what was formeriy the mystery of geographical 

 distribution. Mr. Darwin and others, including Mr. Wallace 

 himself, had found a caudal nexus in the case of i5land>, had 

 shown that the faunas of islands had been derived from that of 

 the nearest mainland, and in a character and degree varyint^ 

 concomitantly with the degree of then: present disconnection 

 therewith. They had thus completed the necessity for "centres 

 of creation." Did not "The Geographical Distribution of 

 Animals afford the requisite evidence for carrjing this com- 

 mencement to its logical conclusion : for sbowmg that in their 

 tum the great continents themselves, or, more precisely, those 



which are outlying to the central mass (which is in the north, 

 around the Pole), have a similar dependence, and have borrowed 

 their own faunas from that northern mass, in a character and 

 degree proportional to the dates and degree of their connec- 

 tion or separation from it, the islands might then be said to be 

 the satellites, and the great zoological regions the planets of this 

 system, all having borrowed their life directly or indirectly from 

 a single "centre of creation." 



To render this still clearer to my own mind I had a map of 

 the worid designed on a polar projection, the northern hemi- 

 sphere being projected to somewhat beyond the southern tropic. 

 By this means the manner in which the land surface of the globe 

 is built around the pole is clearly seen, and the extremities of 

 America, Africa, and Australia, extending into the great oceans 

 of the world, are embraced, or nearly so. When the sub- 

 di\-isional regions (zoologically) of each of these great projec- 

 tions, and of the whole, are marked in colotu^, a succession of 

 zoological strata, to speak rather inaccurately, appears. By 

 carr)-ing an ideal section from the supposed centre of creation in 

 the north through either of these three great extremities, and"from 

 thence to the nearer, and afterwards the more remote, dependen- 

 cies of those extremities (remote not in point of actual distance, as 

 in d^;ree of connection), we pass in each case through zoological 

 strata of different types, until we arrive at those where no land- 

 mammals are to be fonnd at all. And this succession in space, 

 as evidenced by geography, corresponds in a rough way with the 

 succession in time, as revealed by geology, i. As we recede in 

 distance we meet with increased dissimilarity. 2. This dis- 

 similarity partakes of a recession in type. 3, Some of these 

 geographical districts seem to have their counterparts m geo- 

 logical periods. The Ethiopian region, as Mr. Wallace shows, 

 presents us with the exiled miocene fauna of Europe in the most 

 striking naanner. Eocene forms may be seen in its dependenc}' 

 of Madagascar, or in the West Indies. Highly isolated 

 Australia Mith its marsupials, &c., appears as if it were still 

 in the secondary age. Oceanic islands, such as New Zealand, 

 with a more beautiful climate, and more extensive surface than 

 Great Britain, give us no land mammals at all. In others the 

 reptiles "possess the land." 



Mr. Wallace's plan is an excellent illustration of the com- 

 parative method, and shows how a careful classification leads to 

 the solution of historical questions connected with the causes of 

 that classification. Those causes are in this case comprised in 

 the inference that a succession of waves of life has been propa- 

 gated firom the north, not all of which have had an equal 

 extension, nor all encountered similar modifj-ing circumstances. 



If these inferences are not correct, perhaps Mr. Wallace would 

 kindly set me right. J. W. Barry 



I, Duncombe Place, York 



Glaciation of the Italian Lakes 



Having spent some time lately on the border land between 

 Switzerland and Italy, it has occurred to me that a note on some 

 glacial features of that r^ion may prove of interest to the readers 

 of Nature. 



The Lake of Lugano is a rock-basin. I believe it to have 

 been scooped out by the glaciers which have formerly descended 

 from the Alps. Of this there Is abundant proof. The 

 crystalline rocks in their lower reaches possess the easily 

 recognisable outlines of roches vumtontuh, but the stratified 

 mesozoic rocks have lost these characters. Above Lugano and 

 Agno these features are very well marked, and in these localities 

 striation is tolerably frequent, the direction of the striae being 

 southerly. Along both sides of the southern extension of Moimt 

 St. Salvatore to Moreate, striae can be seen in a few places near 

 the lake-level, and the same is the case on both the Pianbello 

 and Generoso shores. At the southern extremities of the lake 

 are abundant moraine-moimds. Erratics are also present, most 

 being gneissose or granitic, but a few have fallen upon the 

 mo\-ing ice from nearer localities, as they are of dolomite. 

 The moraine masses are cut through by the northerly 

 flowing streams, but, after passing the parting between the 

 waters flowing towards Lake Lugano and those running into 

 Lake Como, there is the appearance of great destruction of the 

 moraines. Unfortunately I had a mere cursory glance down the 

 Val della Tresa, through which the drainage of the lake flows to 

 Lago Maggiore. It has often been remarked that in this South 

 Alpine lake district, the dibrh left by the glaciers is exceedingly 

 small when contrasted with similar regions north of that moun- 



