March 6, 1890] 



NA TURE 



413 



and steel, terrestrial magnetism, and the means of 

 obtaining the three magnetic elements. 



In the second chapter, the various modern forms of the 

 mariner's compass, and instruments for adjusting com- 

 passes without sights, are described with illustrations. 

 There is much here which should be of value to com- 

 manders of ships anxious to know as much as possible 

 of their best friend in navigation. 



It is, however, to be regretted that in some particulars 

 both text and illustrations belong to the past, for in Fig. 

 38 an imperfect idea is given of Sir W. Thomson's com- 

 pass. The drawing was correct for 1877, but important 

 improvements were made ten years ago in the substitu- 

 tion of the wire grummet suspension for india-rubber, a 

 change attended with marked success in vessels propelled 

 and severely shaken by powerful engines ; also, in 1881, 

 the adoption of a total reflection prism in the azimuth 

 mirror instead of an ordinary piece of looking-glass. 



Prominence is given to the Hechelmann compass card, 

 which is intended to combine the principles of the 

 Thomson card (which consist chiefly of a long period 

 of oscillation and great lightness), with a much greater 

 magnetic moment in the Thomson-Hechelmann card, as 

 it may be termed. The chief difference in these cards 

 lies in the arrangement of the needles, Hechelmann's 

 idea being to suspend more powerful needles than 

 Thomson's near the circumference, thus bringing the 

 weight as far as possible from the centre of the card to 

 produce a slow period. 



In bringing powerful needles so near the circumference, 

 it is easy to see that something has been lost by Hechel- 

 mann when the quadrantal deviation is to be corrected as 

 it should be — a correction so perfectly accomplished by 

 Thomson. The greater weight of the card, too, tends to 

 increase friction at the cap and pivot. Under these con- 

 siderations the Thomson-Hechelmann card can hardly be 

 considered equal to the modern Thomson. 



In the next chapter, which treats of the magnetism of 

 ships and the resulting deviation, it is satisfactory to find 

 that the different kinds of magnetism which careful 

 investigation has shown to exist in modern vessels are 

 specially mentioned. These are — 



(i) Permanent magnetism. 



(2) Sub-permanent (termed also retentive) magnetism. 



(5) Transient magnetism. 



These definitions are accompanied by a footnote stating 

 that in the English text-books on deviation no difference 

 is made between permanent and sub-permanent mag- 

 netism, but that the two are combined under the expres- | 

 sion sub-permanent. This is perhaps rather hard upon 

 some English books, where, by careful reading, it will be 

 found that the distinction is really made, but, it must be 

 confessed, with a want of that clearness of division 

 which is important to sound knowledge. Readers of 

 the papers published by the Royal Society, and more 

 recently by the Royal United Service Institution, will 

 find that the division of a ship's magnetism into the 

 three kinds mentioned above is strongly insisted upon. 



A complete analysis of the deviations of any given com- 

 pass in a ship, and of the changes which take place on 

 a change of latitude, is necessary before a satisfactory 

 compensation of the deviation by magnets and soft iron 

 can be made. In the " Compass on Board," this analysis 



has a chapter devoted to it, containing information which 

 should be of value both to the captains of ships and com- 

 pass adjusters. It is illustrated by many examples. 



Values of the coefficients v and v' , representing the 

 temporary deviation caused by running on a given course 

 for some days, are given for a number of vessels of dif- 

 ferent types, steam and sailing. They clearly show the 

 navigator of a new ship the need of caution when altering 

 course, and some idea of the amount of change of devia- 

 tion he may expect ; whilst it should be understood that 

 no careful seaman would fail to learn and note the pecu- 

 liarities of the iron affecting his ship's compasses from 

 personal observation under the varied circumstances 

 experienced during each voyage. 



A corrector for the deviation caused by sub-permanent 

 magnetism has yet to be discovered. 



Taking a general view of this book, it may be described 

 as calculated to provide good practical information for 

 the officers of the German mercantile navy, as well as 

 a certain amount of a theoretical nature for those inclined 

 to learn something of a ship's magnetism from a higher 

 standpoint. 



The maps of the three magnetic elements provided at 

 the end of the book are given for the epoch 1885, and on 

 a larger scale than those usually provided in hand-books. 

 The accompanying map of values of the secular change 

 is somewhat open to criticism as regards the figures re- 

 corded in the Red Sea, Bombay, East Indies, and Aus- 

 tralia. This, however, will not prove of any detriment 

 to safety in practical navigation. 



The difficulties connected with the compass in war- 

 ships, with their armoured deck, thickly-plated sides, and 

 conning-towers, are not treated of, and their officers must 

 look elsewhere for the special information they require ; 

 still, there is much to be found in this book that will 

 serve their purpose. 



OUR BOOK SHELF. 



Library Reference Atlas of the World. By John Bartho- 

 lomew, F.R.G.S. (London: Macmillan and Co., 1890.) 



The recognition of the intimate connection that exists 

 between physiography and geography is made very 

 manifest, in all the atlases published during the last few 

 years, by the insertion of maps indicating the physical 

 features of the earth's surface. 



We are in an eminently utilitarian age, and a collection 

 of maps, to meet the requirements of the day, must serve 

 more purposes than that of a mere index to the positions 

 of places ; it must represent the most permanent features 

 of importance in commercial geography, and the dis- 

 tribution of commodities as explained by the sciences 

 of physics, geology, meteorology, biology, &c., or collec- 

 tively by physiography. The elegant work before us 

 satisfies all these requirements, it is as complete as it is 

 a trustworthy atlas of modern geography, and will be 

 equally appreciated by the student, the business man, 

 and the general reader. 



The atlas contains 84 maps, and amongst them we find 

 plates delineating drainage areas, ocean currents, pre- 

 vailing winds, rainfall, temperature, climate, and com- 

 mercial features. A characteristic of the collection is 

 the large number of maps that have been devoted to the 

 British Empire, eighteen plates being given of the United 

 Kingdom alone. India is completed in eight plates, the 

 Dominion of Canada is very completely represented in 

 seven plates, and the mapping of all the British possessions 



