io8 



NATURE 



{Dec. 5, 1889 



analogy points to the probability of there being a decrease 

 in the vertical force in the high latitudes of North 

 America, or the equivalent to a red pole of increasing 

 power repelling the needle for a large area around it. 



The variations in the vertical force at and about these 

 poles or foci of attraction and repulsion at different epochs 

 are not yet sufficiently determined, but if the hypothesis of 

 translation be given up, it is not unreasonable to suppose 

 that the secular changes in the declination and inclination 

 are chiefly dependent upon changes in the relative power 

 of these poles. 



No satisfactory explanation has yet been given of the 

 remarkable changes in the earth's magnetic force as 

 measured on its surface, and suggestions are only possible 

 in the present instance. 



The voyage of the Challenger has shown that local 

 magnetic disturbance is found in the solitary islands of 

 the sea, although surrounded by apparently normal con- 

 ditions, similar to that on the great continents. It has also 

 been suggested that the magnetic portions of these islands 

 causing the disturbance may possibly "have been raised 

 to the earth's surface from the magnetized portion of the 

 earth forming the source of magnetism," and tending to 

 prove Airy's conclusion " that the source of magnetism 

 lies deep." 



In view, therefore, of past geological changes and those 

 now in progress, it may fairly be conceived, not only that 

 large changes have likewise occurred in the distribution 

 of the magnetic portions of the earth appearing here and 

 there on the surface and producing local magnetic dis- 

 turbance, but that there are others of a more progressive 

 character below the earth's surface which are only made 

 manifest by the secular change observed in the magnetic 

 elements. This conception with regard to secular change 

 is not intended to exclude the view that solar influences 

 may have a small share in producing the observed 

 phenomena. 



In conclusion, it may be remarked that they who 

 would fully see the substantial gains to terrestrial magnet- 

 ism which have been obtained by the voyage of the 

 Challenger must refer to the original of this abstract 

 Report, with its plates and charts of the magnetic elements. 

 Subsequent research may add to, qualify, or reverse 

 the conclusions drawn from the observations, but the 

 observations will probably retain a long-abiding value 

 to magneticians. E. W. Creak. 



ON THE SUPPOSED ENORMOUS SHOWERS 

 OF METEORITES IN THE DESERT OF 

 ATA CAM A. 



T T is now universally acknowledged both that meteorites 

 -*■ come from outer space and that shooting-stars, what- 

 ever they are, have an extra-terrestrial origin. It is 

 further asserted that a meteoritic fireball and a shooting- 

 star are only varieties of one phenomenon. Indeed, after 

 it is once granted that a meteoritic fireball is produced 

 by the passage through the terrestrial atmosphere of a 

 dense body entering it with planetary velocity from with- 

 out, and that shooting-stars have an extra-terrestrial 

 origin, it is a very fair assumption that a shooting-star 

 is likewise a dense body rendered luminous during its 

 atmospheric flight. 



One great objection to this assertion is that, again and 

 again, showers of hundreds of thousands of shooting- 

 stars have taken place, during which no heavy body has 

 been observed to reach the earth's surface. The only 

 known case of the arrival of a meteorite during a shooting- 

 star shower has been that of Mazapil, on November 27, 

 1885, and that single coincidence may possibly be the 

 result of accident. A sufficient explanation of this diffi- 

 culty, however, is to be found in the small size of the 

 individuals which produce the appearance of a shooting- 



star shower. That the individuals are really minute is 

 proved by the fact that, while the total mass of a large 

 swarm, like that producing the November meteors, is so 

 small that there is no perceptible influence on the motion 

 of the planets, the number of separate individuals is 

 almost infinite. It is established that the Leonid 

 swarm must be hundreds of millions of miles in length, 

 and some hundreds of thousands of miles in thickness ; 

 and in the densest part of the Bielid swarm, passed 

 through in 1885, the average distance of the individuals 

 from each other was about twenty miles. 



Further, it is now acknowledged that comets are them- 

 selves meteoritic swarms, and Mr. Lockyer has lately 

 brought forward spectroscopic evidence that the fixed 

 stars and the nebulae are similar to comets in their con- 

 stitution. 



The question therefore immediately presents itself. Is 

 the size, of a meteoritic shower, on reaching the earth's 

 surface, ever comparable with that of a meteoritic swarm, 

 as manifested by a shower of shooting-stars .'' 



During the present century nearly 300 meteoritic falls 

 on the earth's surface have been observed, and on only a 

 single date, namely August 25. 1865, has there been 

 observed a fall on two distant parts of the earth on the 

 same day. On that date stones fell at Aumale in Algeria, 

 and at Sherghotty in India ; but as the times of fall differed 

 by about eight hours, and the stones arrived from different 

 directions, it is more than probable that the coincidence 

 of date was accidental. Hence we must infer that a 

 swarm of meteorites, as far as actual observation of 

 tangible objects goes, far from being hundreds of millions 

 of miles long, with individuals a few miles apart, is a 

 comparatively small group, separated from its neigh- 

 bours, if it has any, by a distance comparable with the 

 earth's diameter. 



The extent of surface over which meteoric stones have 

 been picked up after some of the best known and most 

 widely spread falls is given in the following list : — 



Limerick, 3 miles long. 

 Mocs, 3 miles by o"6 mile. 

 Butsura, 3 miles by 2 miles. 

 Pultusk, 5 miles by i mile. 

 L'Aigle, 6 miles by 25 miles. 

 Barbotan, 6 miles long. 

 West Liberty, 7 miles by 4 miles. 

 Stannern, 8 miles by 3 miles. 

 Knyahinya, 9 miles by 3 miles. 

 Weston, 10 miles long. 

 Hessle, 10 miles by 3 miles. 

 New Concord, 10 miles by 3 miles. 

 Castalia, 10 miles by 3 miles. 

 Khairpur, 16 miles by 3 miles. 



As far as I have yet been able to ascertain, the greatest 

 observed separation has been sixteen miles. In the case 

 of Macao, Cold Bokkeveldt, and Pillistfer, wider spreads 

 have been chronicled, but later information has shown 

 the inaccuracy of the earlier statements. 



As regards the meteoric irons, there have only been 

 nine observed falls since the year 175 1 : in seven of them 

 only a single mass was found ; in the remaining two there 

 was in each case a couple of masses, not more than a 

 mile apart. There is thus no recorded instance of an 

 observed shower of meteoric iron. The most convincing 

 proof of the actuality of such showers is furnished by the 

 masses which have been found in the Valley of Toluca, 

 in Mexico ; their existence had been chronicled as early 

 as the year 1784, yet in 1856 it was still possible to col- 

 lect as many as sixty-nine. When etched, they show the 

 Widmanstatten figures in the most excellent way, and in 

 their characters they are typical meteorites. Belonging, 

 as they do, to a single type, they lead to the conviction 

 that they are the result of a single shower. But the 

 region over which the fall took place is not large ; the 



