270 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[July, 



these stations, tlic author enters upon the theoretical invest igation. Tlie 

 fundamental sni>position of the theory of induced magnctisiii, on which Mr. 

 Aiiy states liis calculation to rest, is, that, hy the action of terrestrial magne- 

 tism, every particle of iion is converted into a luagnet, whose directioti is 

 parallel to that of the dijiping ueedlc, and whose intensity is proportional to 

 that of teiTCstrial magnetism, the upper end liaving the property of attracting 

 the north end of the needle, and the lower end that of repelUng it. The 

 attractive and repulsive forces of a particle ou the north eiul of the needle, 

 in the directions of rect-angular axes towards north, towards east, and verti- 

 cally downwards, and of which the compass is t.iken as the origin, are first 

 determined on this su])position in terms of the co-ordinates; and thence the 

 true disturliitig forces of the particle in these dii'ections. The disturliiug 

 forces produced hy the whole of the iron of the ship are the sums of the ex- 

 pressions for every particle. Expressing tliis simnuation hy the letter S. and 

 transforming the rectangular into polar co-ordinates, Mr. Airy gives to the 

 expressions for the disturhing forces the simplifications which tliey admit of, 

 oh the supposition that the compass is in the vertical jilane passing through 

 the ship's keel, and that the iron is systemnietrieally disposed on both sides 

 of that plane. lie thus deduces for the distui'biuing forces acting on the 

 north or marked end of the needle, 



— I cos 5 i\I + I cos 5 P cos 2 A + I sin S N cos A, toward the magne- 

 tic north ; 



I cos B P sin 2 .\ + I sin B N sin A, towards magnetic east ; 



— I sin B Q + I cos S N cos A, .vertically downwards : 



Where I represents the intensity of terrestrial magnetism ; 5 the dip; .\ llie 

 azimuth of the ship's head; and M, N, P, Q, constants depending solely on 

 the construction of the ship, and not changing with any variations of terres- 

 trial localities of magnetic dip or intensity. 



From the consideration of these expressions for the disturbing forces is 

 deduced the following simple rule for the coneetion of a compass disturljed 

 by the induced magnetism only of the iron in a ship. 



1. Determine the ])osition of Barlow's plate with reg.ird to the compass, 

 Trhich will produce the same effect as the iron in the ship. 



2. Fix Barlow's jdate at the distance and depression detennined by the 

 last experiment, but in the opposite azinmth. 



3. Mount another mass of iron at the same level as the compass, but on 

 the starboard or larboard side, and determine its position so that the compass 

 points con'cctly when the shijj's head is N.E., S.E., S.W. or N.W. ; then the 

 compass will be coiTcct iu all positions of the ship's head and in all magnetic 

 latitudes. 



When the disturhing iron of the ship is at the same level as the compass, 

 the correction is stated to be much more simple, it being then oidy necessary 

 to introduce a single mass of iron at the starboard or larboard side, and at 

 the same level as the compass. It is farther remarked, that if one mass of 

 ii'on is placed exactly opposite another equal mass, Ijotli iu azimuth and in 

 elevation, it doubles its distm'bing etfeet : if one mass be placed opposite the 

 other in azinuith, but with elevation instead of depression, or vice versi, it 

 destroys that term of the distm'bance which depends on sin .\, and doidiles tliat 

 which depends on sin. 2 A. And if one mass be placed at tlie same level as 

 the compass, its effects may be destroyed by placing another mass at the 

 same level, in an azinuith ihffering 90'' ou either side. If a disturbance, from 

 whatever cause arising, follow the law of + sin. 2 A, (changing sign in the 

 successive quadrants, and positive when the ship's head is between N. and E.), 

 it may be destroyed Ity placing a mass of ii'on on the starboai'd or larboard 

 side at the same level as the compass ; if it follow the law of + sin. 2 A, tlie 

 mass of iron umst lie on the fore or aft side. From the consideration of tlie 

 expression of the distmbing forces produced liy the ship, it is farther iirferred 

 that both in the construction of the ship and iu the fixing of coiTeciors, no 

 large mass of iron should be placed below the compass. 



■The expressions for the disturbing forces towards north and east, being 

 transformed into forces towards the ship's head and towards the starboard 

 side give 



I cos. 5 ( — M + P) cos. A + I sin. B N, for the former, 

 and 



I COS. i (M + P), for the latter. 



The autliol' next proceeds to investigate the effects which result I'roin the 

 combination of induced magnetism with permanent magnetism. C'aUing H, 

 S, and V the new forces arising from the latter, and directed towards the 

 ship's head, its stai'board side, and vertically downwards, the whole distiu-bing 

 force towards the ship's head becomes 



H + I cos. S (_ M + P) COS. A + I sin. S N ; 

 and the whole disturbing force towards the starboard side, 

 S4-I cos. S (.M + P) sin. A. 



The manner in which the mmierical values of these quantities may be found 

 from experiment is tlicu pointed out, and being determined from the obser- 

 vations on board the Rainbow, at Station I., a comparison is made between 

 the observed disturbances of the needles, and those which wouhl result fiom 

 the action of the ship as a permanent magnet. From this comparison it ap- 

 pears that almost the whole disturbance is accounted for by the permanent 

 magnetism, and that the residual part follows with sufficient approximation 

 the law of changing signs at the successive quadrants. For the complete 

 verification of the theory it remained only to effect an actual correction of the 

 compass. This was done by placing Ijelow the compass, in a position deter- 

 mined by the previously-ascertained numerical values, a large bar magnet to 

 neutralize the effects of the permanent magnetism gf the ship, and a xq\X of 



soft iron on one side of the compass to counteract the distiu'hance arising 

 from induced magnetism. That this correction was effective appears from 

 the very small amount of uncorrected disturbance then observed in tlie com- 

 pass. The observations of the compasses at Stations II., III., IV., are simi- 

 larly discussed : the disturbing force arising from the pcrniauent magnetism 

 of the ship being in like manner determined, a comparison is instituted be- 

 tween the observed and computed disturbance of the compass ; and the 

 results of this comparison, with the exception of the observations at Station 

 IV., are found to be in perfect accordance with the theory, .\ttempts are 

 made to correct the compasses at these stations in the same manner as at 

 Station I., but owing to the imperfection of the compasses they did not suc- 

 ceed so perfectly. The observations made with the dipping needle are next 

 discussed, and the values of the constants .are deduced from them. The 

 general agreement of those determined from the observations when the needle 

 vihr.ated in the direction of the sliip's keel, w ith those deduced from t he obsen'a- 

 tions when the needle vibrated transversely, is pointed out, and is considered 

 an additional proof of the general correctness of the theory. Oliservations 

 on the ihstmbance of the compass in the iron-built sailing-ship Ironsides are 

 next described. These are similar to those in the Rainbow, but not so exten- 

 sive ; and they are discussed on the same principles. From this discussion it 

 is considered that the tlieoiT is in perfect accordance with the facts obsened 

 iu tlie deviations and intensities observed. The coiTcction of one compass 

 was effected by a tentative process, which the author considers likely to be 

 of the highest value in the correction of the compasses of u'on-ships in gene- 

 ral. The sliip's head being placed exactly north, as ascertained by a shore 

 compass, a magnet was placed upon the beam from which the compass was 

 suspended, vsitli the direction of its length exactly transverse to the sliip's 

 keel : it was moved upon the beam to various ilistances till the compass 

 pointed eori'ectly, and then it was fixed. Then the ship's head was placed 

 equally east, and another magnet with its length parallel to the ship's keel, 

 was jilaccd upon the same beam, and moved to different distances till the 

 compass pointed correctly, and then it was fixed. The correction for induced 

 magnetism was ncglectecl, but there would have been no difficulty in adjusting 

 it by the same process, jilacing the vessel's head in azimuth 45^' or 135° or 

 225° or 315°. In conclusion, Mr. .\iry makes the following remarks : — The 

 deviations of the compass at four stations in the Rainbow, and at two stations 

 in the Ironsides, are caused by two modifications of magnetic power; the 

 one being the independent magnetism of the ship, whicli retains, in all posi- 

 tions of the ship, the same magnitude and the same direction relatively to the 

 ship ; the other being the induced magnetism, of wiiich the force varies in 

 magnitude and direction when the sliip's position is changed. In the in- 

 stances mentioned, the effect of the former force was found greatly to exceed 

 that of the latter. It appears that experiments and obsenations similar to 

 tliose applied in the above cases arc sufficient to obtain with accuracy the 

 constants on which at any one place the ship's action on the horizontal needle 

 depends, namelv — 



H S 



;^ + tan S N, r, M, and P ; 



I cos S I cos 5 



and that by placing a magnet so that its action shall take place in a dii'ection 

 opposite to that which the investigations show to be the direction of the 

 ship's independent magnetic action, and at such a distance that its ert'ecti 

 C(|iial to that of the shiji's independent magnetism, and by counteracting the 

 effect of the induced magnetism by means of the induced magnetism of 

 another mass, according to rules which are given, the compass may be made 

 to point exactly as if it were free from distiuiiance. It appears also, that by 

 an easy tentative method, the compass may now be corrccfeil witliout the 

 lahoiur of any numerical investigations or any experiments exceiit those of 

 merely making the f rials. Although the uniformity of the induced magne - 

 f ism under similar circnnistances is to be presumecl, yet the iuvarialiility o 

 the independent magnetism during the course of many years is by no means 

 certain. These statements suggest the following as rules which it is desu'able 

 to obsen'e in the present infancy of iron-ship buihUng. It ajipeai's desirable 

 that — 1. Evei7 iron sea-going ship should be examined by a competent per- 

 son for the accurate determination of the four constants above mentioned for 

 each of the compasses of fhe ship, and a careful record of these determina- 

 tions slioidd be jircserved as a magnetic register of fhe ship. 2. The same 

 person should be cmjiloyed to cxamiiie the vessel at different times, with the 

 view of ascertaining whether cither of the constants changes in fhe course of 

 time. 3. In the case of vessels going to different magnetic latitudes, the 

 same person should make arrangements for the examination of the compasses 

 in other places with a view to the determination of the constant N. 1. The 

 same person should examine and register the general construction of the ship, 

 the jiosifion and cireunistaiices of her building, ivc, with a view to ascertain 

 how far fhe values of the magnetic constants depend on these circnnistances, 

 and iu particular to ascertain their connexion with the value of flic preju- 

 dicial constant M. 5. The same person should sec to the prupcr a|iplication 

 of the corrections and fhe proper measures for preserving the permauency of 

 their magnetism. The most remarkable result in a scientific view from the 

 experiments detailed in the present pa])er is, the gre.it intensity of the per- 

 mauciil magnetism of the uialleable ii'on of which the ship is composed, 



