April 17, 1879] 



NATURE 



553 



methods employed in measuring the distribution and 

 amount of their magnetism. Then come three long 

 chapters recounting very minutely the details of the 

 dimensions, weight, strength, &c., of no less than forty- 

 six individual magnets, together with particulars of the 

 successive magnetisations imparted to them. The work 

 concludes with a discussion of results and of the formulae 

 for empirically representing them, and with a brief 

 obituary notice of the author, by Dr. Figee. 



It appears from the observations of the constructor, 

 van Wetteren, that bars of steel of apparently equally 

 good qualities in other respects will not make equally 

 good magnets ; a point which the author tells us he was 

 unwilling to recognise until he found aU the magnets 

 fabricated from one bar inferior to all the magnets fabri- 

 cated from a bar of what appeared to be equally good 

 steel. English bar steel was found inferior by compari- 

 son with that manufactured on purpose by !M. Wetteren 

 but the author confesses his inability to assign any 

 reason for the inferiority. Concerning the details of 

 forging and tempering a judicious silence is maintained. 

 The method of magnetisation which was found most 

 efficacious both for bar and horse-shoe magnets, was to 

 place their extremities upon the poles of a powerful 

 electro-magnet of the form constructed by Ruhmkorff for 

 diamagnetic experiments ; and then, while thus magne- 

 tised above saturation, to remove them after having 

 applied the appropriate keeper. For magnets weighing 

 so much as half a kilogramme an Elias ring was also 

 applied as an auxiliary in the process of magnetisation. 

 The maximum power was not developed until after two 

 or three such magnetisations, the keeper being momen- 

 tarily removed between each repetition. Reversal of the 

 poles always produced consequent points. The methods 

 of touch, the best of which the author considered to be 

 Hofter's method of stroking the horse-shoe magnet with 

 a second horse-shoe of soft iron from the poles toward 

 the equator of the magnet, he finally rejects, in toto, as 

 being hurtful to the strength and regularity of distribution 

 of the magnetism. 



The most important part of the memoir is that devoted 

 to a discussion of the portative force of magnets. 

 Hacker has given the ratio between the portative force 

 of a horse-shoe magnet and that of a bar-magnet of the 

 same weight and length as two to one. M. van WiUigen 

 found the ratio with an actual magnet of Hacker to be as 

 three to one ; and with van Wetteren' s magnets more 

 than four to one. The empirical formula assigned by 

 Bernoulli to express the relation between the weight of a 

 magnet and its portative force is — 



P = CR\ 



^here p is the weight which the magnet will siistain, R 

 its own weight, and C a coefficient dependent on the 

 quality of steel and other undetermined conditions. A 

 magnet was adjudged good by the author's standard for 

 which Bernoulli's coefficient had a value of 20 or 21 j 

 though 22-5 was occasionally attained. The empirical 

 formula now assigned by van Willigen for the portative 

 force of supersaturated magnetisation is — 



P = aK ^S 



V s'S ' 



and for the permanent portative force — 



where K is the perimeter and S the area of the polar 

 surfaces, / the length of the bar, L the reduced length (or 

 distance between the actual poles or points of maximum 

 free magnetism), and a and /3 two coefficients depending 

 on temperature, quality of steel, temper, &c. It will be 

 seen^at since for magnets of similar form the quantity 

 Ki^'S is proportional to the R^ of Bernoulli's formula, 

 M. van WUligen has determined that factor of the 

 coefficient which is concerned with the length of the 

 magnet and the position of its poles. It would be inte- 

 resting, though out of place here, to compare these results 

 with those recently obtained by M. Petrowchevsky in his 

 researches on the distribution of magnetism in magnets. 



The author falls into the common error of ascribing to 

 M. Jamin the invention of magnets made of laminae of 

 steel bound together in bundles. Magnets of this descrip- 

 tion were employed by Dr. Scoresby in his Arctic explora- 

 tions at the beginning of the century, and may still be 

 seen in the Whitby Museum, where they are deposited. 

 Similar magnets were in even earlier use by Duhamel 

 and Coulomb ; and a magnet almost the counterpart of 

 those of Jamin is described in a memoir on magnets 

 by Geuns pubUshed at Venlo, in Holland, in 1768. 



SiLVANUs p. Thompson 



OUR BOOK SHELF 



Mittheilungen aus detn k. zoologischen Museum zu 

 Dresden, herausgegeben mit Unterstiitzung der konig- 

 lichen Saminlungen fiir Kmist und Wissenschafl. 

 Von Dr. A. B. Meyer. Drittes Heft, mit Tafel 

 XXVI.-XXXV. (Dresden : Baensch, 1878.) 



Dr. Meyer has now issued the third volume of his 

 "contributions" to science from the well-filled stores of 

 the Dresden Museum — a volume which quite equals its 

 precursors in value and interest. The Director first gives 

 us an account of his new cases for the exhibition of zoo- 

 logical objects, and supplies exact details as to their cost. 

 These particulars may be useful for those engaged on the 

 fittings of several other national museums which are now 

 in process of erection. A contribution from M. Edm. de 

 Selys-Longchamps, which follows, contains a general 

 account of the dragon-flies of New Guinea and the 

 Moluccas, and descriptions of a large niunber of new 

 species of these insects. We have next an account of the 

 human skeletons and skulls in the Dresden Museum, 

 dra^vn up by the Director and Herr E. Tungel jointly. The 

 number of skulls in the collection is stated to be 836. 

 We have then an important article by our countryman, 

 Mr. R. Bowdler Sharpe, on the collections of birds be- 

 longing to certain groups, made by Dr. Meyer during his 

 expedition to New Guinea and the Moluccas. The groups 

 treated of in this paper are the Accipitres, Dicruridae, 

 and Campophagidas, of all of which divisions Dr. Meyer 

 obtained a goodly series of specimens, embracing among 

 the Campophagidae examples of nine new species. 



Dr. Kirsch, the Entomologist of the Dresden Museum, 

 follows Mr. Sharpe with descriptions of some new wasps 

 found in the collection, and the volume is concluded by a 

 second portion of Dr. Meyer's memoir on the Papuan 

 skulls of which he obtained such a splendid series during 

 his Eastern Expedition. 



It is quite evident that the present director of the 

 Dresden Museum is not only capable of doing good work 



