Aprils, 1877] 



NATURE 



495 



It is indeed a strange fict that any one of us sitting 

 quietly at his table could, on being told the two numbers 

 just mentioned, draw out a curve on ruled paper, from 

 which thousands of vertical lines might be chalked side 

 by side on a wall, at the distance apart that is taken up 

 by each man in a rank of American soldiers, and know 

 that if the same number of these American soldiers taken 

 indiscriminately had been sorted according to their heights 

 and marched up to the wall, each man of them would find 

 the chalked line which he found opposite to him to be of 

 exactly his own height. So far as I can judge from the 

 run of the figures in the table, the error would never 

 exceed a quarter of an inch, except at either extremity of 

 the series. 



The principle of the law of deviation is very simple. 

 The important influences that acted upon each pellet were 

 the same ; namely, the position of the point whence it was 

 dropped, and the force of gravity. So far as these are 

 concerned, every pellet woirid have pursued an identical 

 path. But in addition to these there were a host of petty 

 disturbing influences, represented by the spikes among 

 which the pellets tumbled in all sorts of ways. The 

 theory of combination shows that the commonest case is 

 that where a pellet falls equally often to the right of a 

 spike as to the left of it, and therefore drops into the 

 compartment vertically below the point where it entered 

 the harrow. It also shows that the cases are very rare of 

 runs of luck carrying the pellet much oftener to one side 

 than the other. The law of deviation is purely numeri- 

 cal ; it- does not regard the fact whether the objects 

 treated of are pellets in an apparatus like this, or shots 

 at a target, or games of chance, or any other of the 

 numerous groups of occurrences to which it is or may be 

 applied.' 



I have now done with my description of the law. I 

 know it has been tedious, but it is an extremely difficult 

 topic to handle on an occasion like this. I trust the 

 application ot it will prove of more interest. 

 {J'o be continued.) 



ON THE STRUCTURE AND ORIGIN OF 

 METEORITES'" 



THE study of meteorites is naturally divisible into 

 several very distinct branches of inquiry. Thus in 

 the first place we may regard them as shooting stars, and 

 observe and discuss their radiant points and their relation 

 to the solar system. This may be called the astronomical 

 aspect of the question. Then, when solid masses fall to 

 the ground, we may study their chemical composition as 

 a whole, or that of the separate mineral constituents ; and 

 lastly, we may study their mechanical structure, and apply 

 to this investigation the same methods which have yielded 

 such important results in the case of terrestrial rocks. 

 So much has been written on the astronomical, chemical, 

 and mineralogical aspect of my subject by those far more 

 competent than myself to deal with such questions, that I 

 shall confine my remarks almost entirely to the mechani- 

 cal structure of meteorites and meteoric irons, and more 

 especially to my own observations, since they will, at all 

 events, have the merit of greater originality and novelty. 

 Time will, however, not permit me to enter into the detail 

 even of this single department of my subject. 



In treating this question it appeared to me very desir- 

 able to exhibit to you accurate reproductions of the natural 

 objects, and I have therefore had prepared photographs 

 of my original drawings, which we shall endeavour to 

 show by means of the oxyhydrogen lime-light, and I shall 

 modify my lecture to meet the requirements of the case, 



' Quetelet, apparently from habit rather than theory, always adopted the 

 binomial law of error, basing his tables on a binomial of high power. It is 

 absoiutelv necessary to ihe theory of the present paper, to get rid of binomial 

 limitalious and to con:ider the law of deviation or error, in its exponential 



=* Abstract of lecture delivered by H. C. Sorby, F.R.S., &C., at the 



Museum, South Kensington, on March 10. 



exhibiting and describing special examples, rather than 

 attempt to give an account of meteorites in general. 

 Moreover, since the time at my disposal is short, and their 

 external characters may be studied to great advantage at 

 the British Museum, I shall confine my remarks as much 

 as possible to their minute internal structure, which can 

 be seen only by examining properly prepared sections 

 with more or less high magnifying powers. 



By far the greater part of my observations were made 

 about a dozen years ago. I prepared a number of sections 

 of meteorites, meteoric irons, and other objects which 

 might throw light on the subject, and my very best thanks 

 are due to Prof. Maskelyne for having most kindly allowed 

 me to thoroughly examine the very excellent series of 

 thin sections, which had been prepared for him. During 

 the last ten years my attention has been directed to very 

 different subjects, and I have done little more than collect 

 material for the further and more complete study of 

 meteorites. When I have fully utilised this material I 

 have no doubt that I shall be able to make the subject far 

 more complete, and may find it necessary to modify some 

 of my conclusions. I cannot but feel that very much more 

 remains to be learned, and I should not hav2 attempted 

 to give an account of what I b,ave so far done, if I had 

 not been particularly asked to do so by Mr. Lockyer. At 

 the same time I trust that I shall at all events succeed in 

 showing that the microscopical method of study yields 

 such well marked and important facts, that in some cases 

 the examination of only a single specimen serves to decide 

 between rival theories. 



In examining with the naked eye an entire or broken 

 meteorite we see that the original external outline is very 

 irregular, and that it is covered by a crust, usually, but 

 not invariably black, comparatively thin, and quite unlike 

 the main mass inside. This crust is usually dull, but 

 sometimes, as in the Stannern meteorite, bright and shin- 

 ing, like a coating of black varnish. On examining with 

 a microscope a thin section of the meteorite, cut perpen- 

 dicular to this crust, we see that it is a true black glass 

 filled with small bubbles, and that the contrast between it 

 and the main mass of the meteorite is as complete as 

 possible, and the junction between them sharply defined, 

 except when portions have been injected a short distance 

 between the crystals. We thus have a most complete 

 proof of the conclusion that the black crust was due to 

 the true igneous fusion of the surface under conditions 

 which had little or no influence at a greater depth 

 than T^xyth of an inch. In the case of meteorites of dif- 

 ferent chemical composition, the black crust has not re- 

 tained a true glassy character, and is sometimes 5\yth of 

 an inch in thickness, consisting of two very distinct layers, 

 the internal showing particles of iron which have been 

 neither melted nor oxidised, and the external showiiigthat 

 they have been oxidised and the oxide melted up with the 

 surrounding stony matter. Taking everything into con- 

 sideration, the microscopical structure of the crust agrees 

 perfectly well with the explanation usually adopted, but 

 rejected by some authors, that it was formed by the fusion 

 of the external surface, and was due to the very rapid 

 heating which takes place when a body moving with 

 planetary velocity rushes into the earth's atmosphere — a 

 heating so rapid that the surface is melted before the heat 

 has time to penetrate beyond a very short distance into 

 the interior of the mass. 



When we come to examine the structure of the original 

 interior part of meteorites, as shown by fractured surfaces, 

 we may often see with the naked eye that they are 

 mottled in such a way as to have many of the characters 

 of a brecciated rock, made up of fragments subsequently 

 cemented together and consolidated. Mere rough frac- 

 tures are, however, very misleading. A much more 

 accurate opinion may be formed Irom the examination of 

 a smooth flat surface. Facts thus observed led Reichen- 

 bach to conclude that meteorites had been formed by the 



