Aprit 2 2, 1886] 



NA TURE 



587 



costly machine-work. As a record of experience up to 

 date, Mr. Warren's paper will have a permanent value. 



The remaining papers on the list are of a miscellaneous 

 character, but all of considerable interest. Mr. Heck de- 

 scribed a " Mechanical Method of Finding the Stability 

 of a Vessel," by means of a simple model. This is a very 

 ingenious and labour-saving device, likely to prove of 

 great assistance in ordinary ship-yards, where a staft" of 

 trained calculators may be wanting. Mr. Stromeyer de- 

 scribed a " Strain Indicator " which he has invented. 

 This instrument is extremely simple in its construction : 

 the essential parts consisting of two flat plates between 

 which is inserted a " rolling-pin " of fine steel wire. Re- 

 lative motion of the two plates causes the rolling-pin to 

 rotate, and its rotation is the means of measuring the 

 strain to which the material is subjected in any portion 

 of a sample or a structure to which the indicator may be 

 attached. If this instrument answers as well as it pro- 

 mises to do, much will be learnt from its indications as 

 to the strains brought upon ships under various conditions 

 and more especially at sea. Such information carefully 

 compiled and collated ought to prove of value in deter- 

 mining the structural arrangements of ships. 



Admiral Paris, the venerable Curator of the Naval 

 Museum at the Louvre, long known for his eminence as 

 a scientific naval officer and as an archsologist in ship- 

 building, attended the meetings, and contributed an in- 

 teresting paper on the " Rolling of Ships," exhibiting an 

 instrument designed to represent the relative movements 

 of ships and waves. His reception was deservedly cordial. 

 Capt. Colomb described, in a well-written paper, some 

 of the more important results of recent measurements 

 of turning powers of ships in the Royal Navy. These 

 trials are now systematised, and much has been learnt 

 from them which will be of value to future naval tactics, 

 as well as useful to shipbuilders in designing rudders and 

 steering-appliances. .A novel steering-gear was described 

 by Mr. Maginnis, who also laid before the Institution 

 some valuable autographic information on the obscure 

 subject of the strains brought upon a rudder when it is 

 " put over " to various angles in a ship moving at speed. 



Mr. Read's contribution, " On the Strength of Bulk- 

 heads " in ships, was seasonable, the recent loss of the 

 Oregon having again drawn public attention to the neces- 

 sity for water-tight subdivisions as a means of safety from 

 foundering. Mr. Read put into a mathematical form the 

 principles which should regulate the construction of bulk- 

 heads if they are to successfully withstand the water-pres- 

 sure which must come upon them when the compartments 

 are " bilged " and sea-water enters. He did not deal with 

 the principles which should govern the disposition of 

 bulkheads ; but these principles are well understood, and 

 more generally acted upon now than formerly. 



Another paper by iVIr. Benjamin described a " Pro- 

 posed Steam Lifeboat " which had been designed to be 

 practically uncapsizable ; and for that purpose, among 

 others, made of a very peculiar form. The only other 

 paper on the list described the improved methods of 

 working anchors and cables devised by the author, Mr. 

 Baxter. This was a paper of a practical and historical 

 character, on a subject of undoubted importance. 



From this hasty summary it will be seen that the 

 Institution of Naval Architects maintained at its recent 

 gatherings its old reputation for widely diversified topics 

 of discussion. And it is to be added that the papers as a 

 whole, numerous as they were, were also of more than 

 average merit. 



ON THE USE OF MODELS FOR INSTRUCTION 



IN THE MAGNETISM OF IRON SHIPS 

 'T'HE deviations of the compass produced by the iron 

 -*- used in the construction of wooden ships was a 

 source of considerable perplexity to the navigators of the 



last and early part of the present centuries; and no sooner 

 were these difliculties fairly overcome thari the building 

 of ships entirely of iron commenced. 



With the introduction of iron ships, prolonged investi- 

 gations into their magnetism and the resulting deviations 

 of the compass on board were undertaken by some of 

 the most eminent philosophers and mathematicians of 

 the day, the subject being still one which occupies the 

 attention of many observers, from the increased use of 

 iron in the equipment, as well as construction, of the 

 hulls and decks. These investigations were much facili- 

 tated by the increased knowledge of the earth's magnet- 

 ism, which received such notable additions from magnetic 

 surveys made by travellers on land and navigators at 

 sea daring the years 1819-45. 



Aloreover, as time rolled on, these observations were 

 embodied in trustworthy gr'aphic representations of the 

 declination or variation, the dip or inclination, and the 

 horizontal force, which have done such good service in 

 the work of obtaining a clear understanding of the cause 

 of the magnetism of iron ships, and the changes to which 

 such magnetism is liable when the vessel's position is 

 altered either geographically or in respect to the magnetic 

 meridian. 



It is not here intended to enter into any historical 

 rhuiiu' of the names of the several investigators in this 

 branch of science, nor of the results which each obtained, 

 but to indicate at once where the physicist and mathe- 

 matician may find the theory and examples of its appli- 

 cation ; also, how the practical results of this elegant 

 theor-y may, by the use of models, be made intelligible 

 and available to the seaman and other inquirers who 

 have neither the time nor the opportunity for abstruse 

 studies requiring considerable mathematical knowledge. 



The text-book which is now generally accepted in all 

 countries is the " Admiralty Manual for the Deviations 

 of the Compass," in Appendix No. i of which will be 

 found the three fundamental equations of Poisson, which 

 form the whole theory of the deviations of the compass, 

 and the expressions of these equations " in terms of the 

 quantities which are usually given and I'equired," written 

 by the late Archibald Smith, M.A., F.R.S. 



The whole of the action of the soft iron of a ship is 

 represented in these equations by the parameters a, h, c, 

 d, e, /, g, li, k, and in a model by nine soft iron rods 

 fixed in definite positions, distinguished by the same 

 letters. 



The effects of the magnetism of the hard iron of the 

 ship are represented in these equations by the parameters 

 P, O, R, and in the model by two permanent magnets 

 held horizontally in definite positions, and a third perma- 

 nent magnet held vertically under the compass. 



One of the most important contributions to magnetical 

 science as regards iron ships was made by Sir George 

 Airy (late Astronomer-Royal) in a paper published in the 

 Phil. Trans. Royal Society for 1839. After making a 

 series of experiments in certain iron-built ships, he dis- 

 cussed the results mathematically with the purpose of 

 discovering a correction for the deviation of the com- 

 pass. He concluded his paper with the announcement 

 of his invention of the system of correction by magnets 

 and soft ii-on, which is universally practised in the pr-esent 

 day, always with advantage, and often as a matter of 

 necessity in "ships of certain types, where to find a suitable 

 place even for the standard compass is a matter of no 

 small difficulty. This system of correction, coupled with 

 the analysis described in the "Admiralty Compass 

 Manual," provides the means of correcting a compass 

 even in position on the 'tween decks of our armour-plated 

 ships of war. 



With these preliminary remarks, the description of 

 some different forms of models will be given, and their 

 uses for inslr-uction in the magnetism of iron ships 

 considered. 



