32 



NA TURE 



[NOVEMIJEK lO, 1898 



Construction for the Direction of a Magnetic Line 

 of Force. 



In Prof. Gray's review of Kiecke's Lehtbuch, a construction 

 for the direction of the line of force at any point, due to a short 

 magnet, is given. 



If the magnet be long, then the following construction 

 holds : 



Let N and S be the two ends of the magnet and P the ])oint, 

 nearer, say, to N than to S. Take A in PS and B in NP pro- 

 duced, so that PA = 

 PB = PN. Take C in 

 UN, so that BC mea- 

 sured towards N e(]uals 

 I'S. Draw CD par.iUel 

 111 PS, D being in the 

 line li.S. Measure PE 

 .Tway from N, so that 

 I'E ^ CD. The dia- 

 gonal PT of the paral- 

 lelogram AE is the 

 direction required. 



J. II. ViNCK.NT. 

 Cavendish Laboratory, 

 Cambridge. 



The determination of 

 the direction of the re- 

 sultant force, at any 

 point, due to a long, 

 thin magnet, is of some 

 importance as a labora- 

 tory exercise, and it is 

 necessary to be able to 

 compare the direction which a small needle takes up in its field 

 with the theoretical direction. For such an exercise the bar 

 should be a long, thin magnet as nearly uniformly magnetised 

 as possible. Kor this case Mr. Vincent's construction gives 

 the theoretical direction very neatly. 



The direction may .ilso be found by dividing the line NS 

 externally, at a point R, say, in the triplicate ratio of NP to SP. 

 The line joining K to P is the direction sought. This construction 

 can be made with only a parallel ruler. 



My friend Mr. G. B. Mathews h.is pointed out tome that this 

 construction may be very conveniently used to draw the whole 

 family of curves. For describe a circle through I' dividing NS 

 internally and externally in the ratio of NP to SP. The lines 

 joining any point on this circle with N and S are in this ratio. 

 1 lencc the direction of the force at each point of the circle is 

 the line joining it with R. Thus, by a succession of circles and 

 corresponding jmsitions of K, the whole series of curves can be 

 laid down. 



The following method is perhaps not so got)d, but is also very 

 easy to remember. A diagram is not necessary. Describe a 

 circle touching the line NP at N, and cutting the line SP pro- 

 duced beyond P in the points II, K, of which II is the nearer 

 to P. From P towards N lay off a distance PL equal to PH, 

 and through L draw a circle touching .SP at S and cutting 

 PN, produced, if necessary, in M. The diagonal passing through 

 P of the parallelogram described on PK, PIVI as adjacent sides is 

 in the direction of the line of force at P. 



When either of the angles SNP, NSP is very obtuse, the last 

 construction shouUl be carried out by drawing the circles so as 

 to give equal segments PH, PL both lying on the side of P 

 towards NS. Then a distance PK'= PK can be laid ofl' along 

 SP produced, and the parallelogram described on PK', P.M as 

 .adjacent sides. There are probably a great many vs'ays of solving 

 this problem : I have hit upon three other distinct methods, 

 which I will not lake up space with describing here. I may 

 mention also that I have given a simple method of laying down 

 successive points on a line of force in my " Magnetism and 

 Electricity, ' vol. i. p. 14, figs. 12 and 13. 



The construction for the direction of the force due to a short 

 magnet, described in my review of Prof. Riecke's book, was 

 given by Ilansteen (" Magnetismus der Erde," s. 20S), and 

 again by Gauss (" \'orschriften," iV:c., Werke Bd. 5, s. 435). It 

 is to be found in Prof. Chrystal's article on " Magnetism " in 

 the " Encyclopaedia Biitannica," and in my treatise on 

 " Absolute Measurements," vol. ii. .\. tiKW. 



NO. 151 5, VOL. 59] 



THE CHEMISTRY OF THE STARS} 

 AXrHEN, on returning from India, I found that you 

 * * had during my absence done me the honour of 

 unaniiTiously electing me your President, I began to cast 

 about for a subject on which to address you. Curiously 

 enough, shortly afterwards an official inquiry compelled 

 me to make myself acquainted with the early doings of 

 the Royal Commission of the Exhibition of 1851, on 

 which I have lately Ijeen elected to serve, and in my 

 reading I found a full account of the establishment of your 

 Institute ; of the laying of the foundation-stone by the 

 late Prince Consort in 1855, and of his memorable speech 

 on that occasion. Here, I thought, was my subject : and 

 when I heard that the admirable work done by this and 

 other local institutions had determined the inhabitants of 

 this important city and neighbourhood to crown the 

 edifice by the foundation of a University, I thought the 

 matter settled. 



This idea, however, was nipped in the bud by a letter 

 which informed me that the hope had been expressed 

 that I should refer to some branch of astronomical work. 

 1 yielded at once, and because I felt that I might thus be 

 able to show cause why the making of knowledge should 

 occupy a large place in your new University, and thus 

 distinguish it from other Universities more or less 

 decadent. 



The im])ortance of practical work, the educational 

 value of the seeking after truth by experiment and 

 observation on the part of even young students, are now 

 generally recognised. That battle has been fought and 

 won. 15ut there is a tendency in the official direction of 

 seats of learning to consider what is known to be useful, 

 because it is used, in the first place. The fact that the 

 unknown, that is the unstudied, is the mine from which 

 all scientific knowledge with its million applications has 

 been won is too often forgotten. 



Bacon, who was the first to point out the importance 

 of experiment in the physical sciences, and who predicted 

 the applications to which I have referred, warns us that 

 "lucifera experimenta non fructifera quaerenda": and 

 surely we should highly prize those results which enlarge 

 the domain of human thought and help us to understand 

 the mechanism of the wonderful universe in which our 

 lot is cast, as well as those which add to the coinfort 

 and the convenience of our lives. 



It would be also easy to show by many instances how 

 researches, considered ideally useless at the time they 

 were made, have been the origin of the most tremendous 

 applications. One instance suffices. Faraday's trifling with 

 wires and magnets has already landed us in one of the 

 greatest re\olutions which civilisation has witnessed ; 

 and where the triumphs of electrical science will stop, 

 no man can say. 



This is a case in which the useless has been rapidly 

 sublimed into utility so far as our material wants are 

 concerned. 



I propose to bring to your notice another "useless" 

 observation suggesting a line of inquiry which I believe 

 sooner or later is destined profoundly to influence human 

 thought along many lines. 



F'raunhofer at the beginning of this century examined 

 sunlight and starlight through a prism. He found that 

 the light received from the sun differed from that of the 

 stars. So useless ditl his work appear that we had to 

 wait for half a century till any considerable advance was 

 made. It was found at last that the strange "lines" seen 

 and named by Fraunhofcr were precious indications of 

 the chemical substances present in worlds immeasurably 

 remote. We had, after half a century's neglect, the 

 foundation of solar and stellar chemistry, an advance 

 in knowledge equalling any other in its importance. 



1 An inaugur.1l address oelivcrcd .-it the Birminehiim .-ind Midland Insti- 

 luieon October aS, by Sir Norm.n Lockycr, K.C.B., F.R.S., President. 



