128 



NATURE 



\_May 30, 1878 



St. Helena 16 S. ..- 22 S. ... 4-8 



Cape of Good Hope 34,, ... 53.. ••• 4*6 



Hobarton ' 43 .. ••• 7° ,, ••. 4'3 



It will be seen that in the months in which the sun's 

 action is a maximum/ for each hemisphere the east and 

 west movements of the needle in its true position (that 

 which is independent of gravity) do not vary with lati- 

 tude ; the maximum range appears, in fact, to take place 

 near the tropics, and when the sun is in the zenith. 



If, however, we should prefer to consider the oscilla- 

 tions of the horizontal needle due to the direct action of 

 electrical currents upon it rather than upon the earth, we 

 must remember the change of the force which directs 

 the needle as we proceed from one latitude to another. 

 If we wish to compare the vertical heights through which 

 a body will fall in a second of time on an inclined plane 

 at two stations, we must take into account not only the 

 force of gravity at each station, but also the angle which 

 the inclined plane makes with the horizontal plane. 

 When we employ the same unit of directive force at the 

 stations in the north hemisphere for the month of 

 August, we obtain the following comparative ranges : — 



Makerstoun ... 4*3 



Toronto ... 4'8 



Simla 4*4 



Bombay ... 4*6 



Madras 3*8 (September = 4-2). 



The values are less for Trevandrum and St. Helena, 

 but there is no appearance of a law which can be referred 

 to latitude ; and there is no way in which we can examine 

 the question which will satisfy such a relation. If we 

 take any zone or zones of the earth which will include as 

 much of the northern as the southern hemisphere, the 

 mean movement for them will be nearly zero, on account 

 of the opposite directions of the oscillations ; it is for 

 this reason that there is a diminution of range, especially 

 in the months near the equinoxes, for equatorial stations. 

 On the whole the conclusion is, that the diurnal law of 

 oscillation east and west of a magnetic needle is nearly 

 the same in all latitudes for a given position of the sun 

 and a given directive force. The deviations from this 

 rule are connected with magnetic disturbances which 

 have most effect near the poles, and with the opposition 

 of forces near the equator. We have thus to deal with 

 a phenomenon which is little dependent upon local 

 causes, and which may, in its great features, be con- 

 sidered cosmic. 2 



These facts understood, we are now in a better position 

 to consider the great change in the range of this oscilla- 

 tion, which occurs in the decennial and sun-spot period. 

 Let us examine what that change really is. It does 

 not matter here whether we refer to the motion of the 

 horizontal or of the dipping needle; we find that if 

 the mean range is counted 10 in England in the years 

 when it is a minimum, the years of fewest spots, then it 

 becomes 16 or thereby when it is a maximum, that is, in 

 the years for which the sunspots are most numerous. 



Now this great change in the effect of the solar action is 

 felt in nearly the same way, and to the same proportionate 

 amount, all over the globe. The law of the oscillation 

 is not changed; the needle attains the most westerly 

 position in one hemisphere, and the most easterly in the 

 other, at the same hour as before, but the oscillation is 

 nearly sixty per cent, greater at Hobarton in Van Diemen 

 Island, at Trevandrum on the magnetic equator, at 

 Toronto in Canada, and in England. 



When the observations have been continued sufficiently 

 long with equal care, we can find that the ratio of the 

 maximum range to the minimum is undergoing, at some 



" The month of maximum varies within the tropics : at Madras it is in 

 September, and for that month the range multiplied by cos. dip — 5' "3 

 nearly. _ . ., ,, 



2 For these reasons no such inverse relation with latitude exists as M. Faye 

 has supposed between the diurnal oscillation of the magnetic needle and of 

 the barometer. 



stations at least, a slow change. Thus, at Trevandrum 

 the successive ratios of the maximum of i860 and 1870 

 to the preceding and following minima are — 



^S6o __^ i860 ^.__ 1870 1870 



I856' ^^' 1866' ^^ ' Im' ^^ ' 1877' 



1-51. 



It must be remembered that the thesis which M. Faye 

 supports is, that the diurnal oscillations of the magnetic 

 needle are due to the solar heat, and that he has shown 

 that no appreciable change of temperature is due to the 

 spotted surface of the sun. We may ask then, Where 

 are we to find the change of temperature which causes 

 so great a variation in the sun' s action ? We need not 

 calculate the difference of temperature between the 

 photosphere and the nucleus of a spot, and we need not 

 theorise on the possible difference between the solar 

 radiation when there are few and many spots ; we have 

 got thermometers; we have even observations of the 

 evaporation of water, from which solar action M. Faye 

 finds the atmospheric electricity which should produce 

 the magnetic variations. What do they say ? If effects 

 have any relation whatever to their causes, surely when 

 the effect of the solar action in producing the diurnal varia- 

 tions of the earth's magnetism is increased by a half or 

 three-quarters of its value from the time of most spots till 

 the sun shines with unspotted surface, we should expect 

 some marked changes of temperature from year to year, 

 if change of temperature is in question. I have already 

 remarked that no such change of temperature has been 

 found. 



M. Faye says with reference to the question, What is 

 the cause of the decennial magnetic variation ? " The 

 question would perhaps be embarrassing if we had only 

 that to ask, but the elements of terrestrial magnetism pre- 

 sent other variations which are as much independent of 

 the sun-spots. Such are the secular changes which dis- 

 place gradually the magnetic poles of our terrestrial 

 globe. We must seek the cause not in the heavens but 

 in the slow modifications of which the earth' s surface is 

 the theatre. They are due probably to the works of men 

 and above all to the continued action of geological 

 forces." Of course these ideas refer to the secular 

 changes, even for which they will not be readily accepted, 

 but they do not touch on the fact that an explanation is 

 offered, that of the solar heat, for the diurnal variations, 

 and that no evidence is produced that the supposed cause 

 undergoes any change in the decennial period. 



I have referred only to oscillations of the magnetic 

 needle, which may be considered due to the variations of 

 an easterly force ; but the force of the earth' s magnetism 

 which directs the needle north and south, obeys also a 

 law of diurnal variation, and the range of this variation 

 follows the same law in the decennial period as that for 

 the ranges of the oscillations. Thus at Trevandrum, near 

 the magnetic equator, the ran^e of the diurnal variation 

 of the total force of the earth's magnetism in the year 

 for which it was a maximum, was to the range in the 

 year of minimum as 177 to 10. The corresponding ratio 

 of the ranges for the oscillations of the needle having 

 been as 15-9 to 10.^ 



As there is not the slightest evidence of a decennial 

 variation in the solar heat, and as there is an absolute 

 certainty that if any variation exists it is of an amount so 

 very small that it could not account for the great changes 

 in the magnetic variations, the conclusion appears to me 

 inevitable, that these variations are not due to the solar 

 heat. We fortunately possess the means of deciding 

 this question by the study of phenomena due to our 

 satellite, whose heating action M. Faye has shown to be 

 quite inappreciable. John Allan Broun 



{To be continued^ 



^ I would remark here that the epoch of maximum range of foiice was not 

 exactly the same as that for the maximum oscillation, a fact which may not 

 be without importance when the mode of solar action is invesugateo. 



