128 



NATURE 



[Dec. 12, 1878 



first method. In the Reports of the Liverpool Compass 

 Committee and in Mr. Towson' s " Information for 

 Masters and Mates regarding Ship's Magnetism," in- 

 stances of perplexing changes in the compass are given, 

 and are referred to the same cause. The " sluggishness ' ' 

 of ship's magnetism, according to which it depends 

 generally in part on the influence experienced some time 

 before the time of observation, and not wholly on the 

 influence at the time, seems to have been first definitely 

 noticed and discussed scientifically by Sir Edward Sabine 

 in his analysis of the results of the magnetic observations 

 in the Antarctic Exploring Expedition of Sir James Ross 

 in the Erebits and Terror, in the years 1840-41. 



The practical rule and caution given above is of great 

 importance in the navigation of iron ships. The amount 

 of the error which may be found cannot be predicted for 

 ships in general, nor for any particular ship except after 

 much experience and careful observation. A small effect 

 of two or three degrees,^ such as that referred to in the 

 Admiralty Manual as found in M. Gaussin's experience, 

 may be observed in the course of quietly swinging a ship 

 by hawsers or steam-tugs. If the ship under way is 

 steamed round on the different courses the amount of the 

 "Gaussin error" may generally be greater than if she 



is hauled round by warps ; but we must not be sure 

 that it will be so, because the shake of the screw which 

 enhances the magnetisation on the east or west courses 

 may shake it out again before the observation is made on 

 the north or south courses. 



A good practical rule in correcting the compass is, after 

 having got it quite correct on the north and south courses, 

 correct just half the error which is found after that on the 

 south and north course, in the regular swinging of the 

 ship. 



The warning at the head of this article is particularly 

 important for ships of war after firing guns when on 

 easterly or westerly courses ; if the course is then changed 

 to north or south, and particularly if, after the firing, the 

 change of course is effected under canvas, without the 

 shaking of the ship's magnetism produced by the engines 

 and screw. 



The warning is also very important for ships steaming 

 through the Mediterranean eastwards or westwards, and 

 then turning south through the Suez Canal or north 

 round Cape St. Vincent ; and for ships steaming east- 

 wards from America and then turning northwards or 

 southwards into St. George's Channel. 



MA THE MA TICAL DRA WING INSTR UMENTS ^ 



T N his preface the author states that we nowadays ex- 

 -■• pect to find somewhere in print an account of the 

 little mysteries of any particular art, and that partly with 

 the hope of enabling this expectation to be fulfilled, and 

 partly to meet the constant inquiries made respecting 

 certain of the more complicated instruments manufac- 

 tured by him, he has written his book. The author offers 

 as an apology for any shortcomings in his work, that he 

 is conscious his powers are greater with the lathe and 

 file than in the ways of gentle rhetoric. In our opinion 

 this is unnecessary ; we would rather have had the file 

 marks more distinct, and the technical details of con- 



struction not so carefully polished out in the work before 

 us as in the well-finished instruments for which the author 

 is so well known. The drawing instruments in most 

 common use, pen, compasses, and dividers, are first de- 

 scribed, and the patterns most recommended are illus- 

 trated ; the type of these instruments now in use seems t& 

 meet all requirements, and if of the best quality, appears 

 to require little or no improvement ; the needle-point, 

 however, shown on p. 34 (Fig. i), adapted to compasses or 

 pricker, is an improvement in steadiness on the old form^ 

 which was always liable to a little play. The earlier 

 chapters will probably be of use to the beginner in facili- 

 tating his choice of the requisite instruments for his 

 work, but he must recollect that dexterity in their use. 



SECTION. FBOyir CF EOINT 



Fig. I. 



even if of the most improved form, can only be acquired 

 with very considerable practice ; much time and temper 

 may be saved by the use of good instruments, and there 

 is nothing particularly meritorious in the production of 

 good work with bad instruments if good ones are within 

 his reach. 



Of the more complicated instruments next described, 

 some must be regarded rather as mechanical curiosities 

 than of every-day use ; others, however, are indispensable 

 where accuracy and the saving of time are of importance ; 

 as chief among these we select the eidograph and plani- 

 meter. A plan can be reduced or enlarged by dividing it 



' Much greater effects than this are actually found in the cases of gun- 

 practice and of long steaming on easterly or westerly courses referred to 

 Del 



elow. 



^ " Mathematical Drawing Instruments 

 (London : E. and F. N. Spon, 1878.) 



by William Ford Stanley. 



Fig. 2. 



into small squares and filling the details contained ii> 

 each square into the corresponding squares ruled on 

 the sheet prepared for the copy. This is a tolerably 

 rapid process w'hen the plan is simple in character, and 

 with the help of proportional compasses a good draughts- 

 man may attain considerable accuracy, but for a compli- 

 cated plan or where great exactness is required, either the 

 pentegraph or eidograph is indispensable. The author 

 justly expresses astonishment at the little use at present 

 made of the eidograph as compared with the pentegraph ; 

 the latter as made in this country appears for large work 

 a most clumsy contrivance, offering much unavoidable- 

 resistance to motion, and even if made on the most im- 

 proved Continental pattern is much less handy than the 

 former. The eidograph, as improved by the author, is 

 shown in the cut (Fig 2). 



