Feb. 5, 1874J 



NA TURE 



269 



water. It sometimes lose to great distinctness, and sometimes 

 fell to inaudibility. These fluctuation.s, of which various striking 

 examples have been observed, are due to the drifting of acoustic 

 clouds, which act upon a source of sound, as the drifting of 

 ordinary clouds upon the sun. The whi-tle showed the same 

 intermittence as to period, but in the opposite sense, for when the 

 wliistle was faint the pipe was strong, and vice vend. 



There seemed to be an extraordinary amount of sound in the 

 air. It was filled with a resonant roar from the Bayswater and 

 Kniglitsbridge roads. The railway whistles were extremely dis- 

 tinct, while the fog-signals exploded at the v.irious metropolitan 

 stations kept up a loud and almost constant cannonade. I could 

 by no means reconcile this state of things with the statements so 

 categorically made regarding the influence of fog. 



The water was on this day warmer than the air, and the 

 ascending vapour was instantly in part condensed, thus revealing 

 its distribution. Instead of being uniformly diffused, it formed 

 wreaths and stria". I am pretty confident that had the vapour 

 been able to maintain itsell as such, the air would have been far 

 more opaque to sound. In other words, I believe that the 

 very cause which diminished the optical transparency of the 

 atmosphere augmented its acoustical transparency. 



This conclusion was confirmed by numerous observations made 

 while the fog lasted. * 



On Dec. 13 the fog was displaced by a thin haze. We could 

 plainly see from one bank of the SL-rpentine to the other, and far 

 into Hyde Park beyond. There was a wonderful subsidence of 

 the sound of the carriages, church bells, &c. Being at the 

 bridge I listened for the sounds excited at the end of the Ser- 

 pentine. With the utmost stretch of attention I could hear 

 nothing. I walked along the edge of the water towards Mr. 

 Cottrell, and when I had lessened the distance by one half, the 

 sound of his whistle was not so distinct as it had been at the bridge 

 on the day of the densest fog. Hence the optical cleansing of the air 

 by the melting of the fog had so darkened it acoustically, tliat a 

 sound generated at the end of the Serpentine was lowered to at 

 least one-fouTth of its intensity at a point midway between the 

 end and the bridge. 



This opportune fog enabled me to remove the last of a con- 

 geries of errors which, ever since the year 1 70S, have attached 

 themselves to this question. As regards phonic coast-signals, we 

 now know exactly where we stand. 



It is worth observing he:e that the solution of the deportment 

 of hail, rain, snow, haze, and fog, as regards sound, depends 

 entirely upon observations made on the 3rd of July, which was 

 about the last day that one would have chosen for experiments 

 on fog-signals. Indeed, it had been distinctly laid down that 

 observations on such a day would be useless ; that they might 

 indeed enab'e us to weed away bad instruments from good ones, 

 but could throw no light whatever on the question of fog- 

 signaling. That the contrary is the case, is an illustration of the 

 fact that the solution of a question often lies in a direction dia- 

 metrically opposed to that in which it appears to lie. t 



EXJR.-iCTS FROM AN ADDRES.S BY SIR IV. 

 THOMSON, TO THE SOCIETY OF TELE- 

 GRAPHIC ENGINEERS 



T HAVE advisedly, not thoughtlessly, used the expression 

 ■'- " terrestrial electricity." It is not an expression we are 

 accustomed to. We are accustomed to "terrestrial mannetisra ; " 

 We are accustomed to atmospheric electricity. The electric 

 telegraph forces us to combine our ideas with reference to 

 terrestrial magnetism and atmospheric electricity. We must 

 look upon the earth and the air as a whole— a globe of earth 

 and air — and consider its electricity whether in rest or in motion. 

 Then, as to terrestiial magnetism, of what its relation may be to 

 perceptible electric manifestations we at present know nothing. 



You all know that the earth acts as a gieat magnet. Dr. 

 Gilbert, of Colchester, made that clear nearly 300 years ago ; 

 but how tlie earth acts a^ a great magnet — how it is. a magnet, — 

 whether an eh ctro magnet in virtue of currents revolving round 

 under the upper surface, or whether it is a magnet like a mass 

 of steel or load-stone, we do nut know. This we do know, that 



* Since the first notices of this lecture appeared in the newspapers, strong 

 confirmatory evidence has been received. 



t The loregoing report w.i5 compiled from the notes of Prof. Tyndall. It 

 is published with Prof. Tyndall's sanction, but was not written by himself. 



it is a variable magnet, and that a first approximation to tlie 

 variation consists in a statement of motion round the axis of 

 figure — motion of the magnetic poles, round the axis of figure, 

 in a period of from 900 to 1,000 years. The earth is not a 

 uniformly magnetised magnet with two poles, and with circles 

 of symmetry round those poles. But a first expression — as we 

 should say in mathematical language the first " harmonic term " 

 — in the full expression of terrestrial magnetism is an expression 

 of a regular and symmetrical distribution such as I have indi- 

 cated. Now, this is quite certain, that the axis of this first term, 

 so to speak, or this first approximation, which, in fact, we 

 might call tlie magnetic axis of the earth, does revolve round 

 the axis of figure. 



When the phenomena of terrestrial magnetism were first some- 

 what accurately observed about three hundred years ago, the 

 needle pointed here in England a little to the east of north ; a 

 few years later it pointed due north ; then, until about the year 

 1S20, it went to the west of north ; and now it has come back 

 towards the north. The dip has experienced corresponding 

 variations. The dip was first discovered by the instrument 

 maker, Robert Norman, an illustration, I may mention in 

 passing, of the benefits which abstract science derives from 

 practical applications — one of the most important fundamental 

 discoveries of magnetism brought back to theory by an instru- 

 ment maker who made mariner's compasses. Robert Norman, 

 in balancing his compass cards, noticed that after they were 

 magnetised one end dipped, and he examined the phemonenon 

 and supported a needle about the centre of gravity, magnetised 

 it, and discovered the dip. When the dip was first so discovered 

 by Robert Norman it was less than it is now. The dip has gone 

 on increasing, and is still increasmg ; but about 50 years ago the 

 deviation from true north was greatest. Everything goes on as 

 if the earth had a magnetic p(jle revolving from west to east 

 round the true North Pole, at a distance of 20° from it. About 

 three hundred years ago its azimuth from t ngland was a little to 

 the cast of the north pole : then it came round, moving eastwards 

 on the far side of the north pole, and round in a circle towards us 

 on the left-hand side of the north pole, as looked to from England. 

 That motion in a circle round the north pole has already been 

 experienced within the period during which somewhat accurate 

 measurements have been made — has been experienced to the 

 extent of rather more than a quarter of the whole revolution ; 

 and we may expect that about 200 years from the present time 

 the magnetic pole will be between England and the North Pole ; 

 so that the needle will thus point due north, and the dip be 

 greater than it has been for 1,000 years, or will be for another. 

 It is one of the greatest mysteries oi science, a mystery which 1 

 might almost say is to myself a subject of daily contemplation — 

 what can be the cause of this magnetism in the interior of 

 the earth? Rigid magnetisation, like that of steel or the load- 

 stone, has no quality in itself in virtue of which we can conceive 

 it to migrate round in the magnetised bar. Electric currents 

 afford the more favoured hypothesis ; they are more mobile. 

 If we can conceive electric currents at all, we may conceive 

 them flitting about. But what sustains the electric currents? 

 People sometimes say, heedlessly or ignoranlly, that thermo- 

 electricity does it. We have none of the elements of the problem 

 of thermo-electricity in the state of underground temperature 

 which could possibly explain, in accordance with any knowdedge 

 we have of thermo-electricity, how there could so be sustained 

 currents round the earth. And if there were currents round the 

 earth, regulated by some cause so as to give them a definite 

 direction at one time, we are as far as ever from explaining how 

 the channel of those currents could experience that great secular 

 variation which we know it does. Thus we have merely a 

 mystery. It would be rash to suggest even an explanation. I 

 may say that one explanation has been suggested. It was sug- 

 gested by the great astronomer, Halley, that there is a nucleus 

 in the interior of the earth, and that the mystery is explained 

 simply by a magnet not rigidly connected with the upper crust 

 of the earth, but revolving round an axis differing from the axis 

 of rotation of the outer crust, and exhibiting a gradual precessional 

 motion independent of the precessional motion of the outer rigid 

 crust. I merely say that has been suggested. I do not ask you 

 to judge of the pr.ibability : I would not ask myself to judge of 

 the probability of it. No other explanation has been suggested. 



But now, I say, we look with hopefulness to the practical 

 telegraphist for data towards a solution of this grand pioblcm. 

 The terrestriil magnet is subject, as a whole, to the grand secular 

 variation which I have indicated. But, besides that, there aie 



