3Q 



NATURE 



[May 10, 1888 



trustworthy information. For some seventeen years I have kept 

 n. complete list of all salt exports from the Mersey ports, and this 

 list, I think, is the only complete one published, though the Salt 

 Chamber of Commerce here professes to have a list, which it 

 does not issue for public use. Indeed, I regret to say that it is 

 almost impossible to get any assistance or information from this 

 body. 



The French Government issues at times a list of salt manu- 

 factured or raised from mines. The last I received, viz. 1879, 

 gave, as the production for that year, 283,000 tons of sea salt and 

 293,000 tons of rock salt. 



I shall be glad to give any information I possess to Mr. Merrill, 

 and should be glad if any of your readers could give any in- 

 formation or assistance that would enable me to make as complete 

 as possible the Salt Museum we are here forming. 



Northwich, Cheshire. Thomas Ward. 



Prof. Rosenbusch's Work on Petrology. 



Prof. Bonney's letter (Nature, vol. xxxvii. p. 556) makes me 

 venture once more to ask permission for space for a few remarks. 

 One of the objects I had in view in writing to you at first is 

 partly attained by the appearance of Prof. Bonney's " friendly 

 protest" ; and his remark that but for my letter he should have 

 refrained "for a season" leads me to hope that in due course 

 this object may be still further realized. 



Prof. Bonney sees great objections to Rosenbusch's system of 

 classification, and demurs to some of his groups altogether, both 

 as to those admitted and those omitted. Naturally, then, he 

 desires that this system shall not, by students of petrology, be 

 too readily accepted nor too blindly followed. I do not think 

 there is much danger of this, nor do I think that the ' ' viaduct " 

 was too much complimented either by Dr. Hatch or myself, the 

 defective foundation of the piers in question being quite 

 sufficiently alluded to for the time being. 



The position, however, seems to be this. The number of 

 earnest students of petrology is larger now than formerly, and 

 is on the increase. They feel that no satisfactory system of 

 classification had yet been offered toth^m, and indeed are rather 

 bewildered by the fact that opinions as to what is the best 

 system have been almost as many in number as the teachers 

 who could by any means claim to be authorities entitled to 

 instruct in this matter. Also, it is now a long time since any 

 detailed system of classification, covering the whole ground, 

 has been attempted. 



Now we have such an attempt offered to us by Rosenbusch, 

 and there is no doubt that to many it will be very welcome and 

 will be largely used, in spite of the defects undoubtedly seen in it. 



Prof. Bonney objects to the viaduct because of the weakness 

 of some of its piers, and still more strongly objects to it, I think, 

 because he considers that when a student has crossed it he will 

 arrive at a point from which he will obtain a view of the sur- 

 rounding country which will not be a good or correct view, and 

 which will in some respects confuse the knowledge of that country 

 already obtained and still to be sought for. 



Would not this be just exactly the best time for some 

 authority of great experience to come forward and point out to 

 us younger workers wherein the viaduct is defective, and wherein 

 we shall see wrongly from the ground on the further side of it ; 

 and to tell us his opinions as to a better viaduct, so placed as to 

 lead us to a better point of outlook ? 



May we hope that Prof. Bonney will himself give us such a 

 detailed criticism of the subject ? It would be received with 

 great attention and gratitude by many who, like myself, are 

 looking for "light and leading" in this branch of study. 



A. B. 



History of the Contraction Theory of Mountain 

 Formation. 



In his "Physical Geology," second edition, p. 674, Prof. 

 Green says : "The notion that the earth's contraction has been 

 the cause of the displacement of the rocks and the elevations 

 of the surface seems to have occurred first to Descartes (ed. 

 francaise, 1668, p. 322)." 



It does not seem to be generally known that, a few years 

 later, the same idea occurred to Newton. In a letter to Dr. 

 Thomas Burnet he refers to that writer's "Sacred Theory of the 

 Earth," the Latin edition of which was published in 1681, and 

 considers the creation of the earth in connection with the Mosaic 

 account. After suggesting illustrations of the "generation of 



hills," Newton concludes thus: "I forbear to describe other 

 causes of mountains, as the breaking out of vapours from below 

 before the earth was well hardened, — the settling and shrinking 

 of the whole globe after the upper regions or surface began to be 

 hard;" though he adds, " I have not set down anything I have 

 well considered, or will undertake to defend." 



The letter, which is written in reply to one of Burnet's, dated 

 January 13, 1680-81, is given in full in Brewster's " Memoirs of 

 Sir Isaac Newton," vol. ii. Appendix 4. The manuscript from 

 which it is printed is a copy of the letter, without date or 

 signature ; but, according to Brewster, "the whole is distinctly 

 written in Sir Isaac's hand." Charles Davison. 



Birmingham, April 23. 



Lightning and Milk. 



Emin Pasha (Nature, vol. xxxvii.. p. 583) mentions the 

 African superstition "that fire kindled by a flash of lightning 

 cannot be extinguished until a small quantity of milk has been 

 poured over it." This idea is embodied in a Russian proverb, 

 and has also existed in parts of Germany (Boyes, Lacon, 

 p. 157). Emin Pasha adds that, in tempering swords made 

 from meteoric iron {vulgo, thunderbolts), the blacksmith uses 

 not water, but milk. Are other instances of this custom known ? 

 Has any explanation been offered ? Indian folk-lore furnishes 

 two ideas which may illustrate it : one, that the fall of a meteor is 

 a bad omen {Indian Notes and Queries, July 1887, 674) ; the 

 other, that evil spirits are very fond of fresh milk (ib., Decem- 

 ber 1886, 198). Meteorites and lightning are connected in the 

 minds of ignorant people, particularly, as Emin Pasha tells us, 

 in the present instance. The milk, therefore, whether applied 

 by smith or fire-man, may be rather intended as a propitia- 

 tion than used for its intrinsic power of tempering steel or 

 extinguishing flame. F. A. BATHER. 



20 Campden Hill Road, Kensington, W., April 29. 



The Duplex Pendulum Seismograph. 



As the accuracy of the duplex pendulum seismograph has 

 been impugned by a writer in Nature, vol. xxxvii. p. 571, 

 who at the same time adopts the instrument (with modifications 

 which are, in my opinion, the reverse of improvements) I forward 

 to you comparison diagrams. They show side by side the record 

 given by the seismograph itself, and the real motion of the base 

 of the instrument when that was artificially shaken in a manner 

 that closely imitated an earthquake. The real motion was re- 

 corded by means of a multiplying lever hinged by a universal 

 joint in a bracket fixed to a separate support. In both records 

 the motion is magnified about six times. The agreement of the 

 two demonstrates the accuracy of the instrument as an earthquake 

 recorder, alike for large and for small motions. These are ex- 

 amples of tests which I have been in the habit of applying to 

 seismographs since 1880 (see Proc. R. S., vol. xxxi. p. 440). In 

 the present case the test was made with one of the duplex 

 pendulum seismographs made and sold by the Cambridge 

 Scien'ific Instrument Company, and described by me in Nature, 

 vol. xxxiv. p. 343. J. A. EwiNG. 



University College, Dundee, April 20. 



Self- Induction. 



I have to apologize for erroneously attributing to Dr. Lodge 

 a suggestion with reference to the self-induction of wires for 

 high-tension electric discharges. I do not, however, consider, 

 as Prof. Lodge appears to do, that for such dis harges it is "on 

 the face of it absurd" to suppose that the self-induction of iron 

 wires is less than that of copper wires of the same dimensions. 

 Prof. Ewing has suggested that for very small values of the 

 magnetizing force, H, iron may p;s~ibly behave as a diamagnetic 

 body, and the corresponding values of the magnetic sus- 

 ceptibility, k, may be negative. The values of the magnetic 

 induction, B, which are given by the equation — 



B = (1 + 4ir£)H, 

 will be less than H, because k is negative. The rate of increase 

 of B with H will be less than unity for iron if this supposition 

 is true, and will be equal to unity for copper, for which we may 

 suppose that the value of k is negligible. The coefficient of self- 

 induction, which will be proportional to the rate of increase of 

 B with regard to H for wires of the same dimensions will 

 accordingly be less for the iron than for the copper. 



City and Guilds Institute, May 2. W. E. Sumpner. 



