592 



NATURE 



[April 21, 1 88 1 



fracture of the earth's crust which runs in a south-westerly 

 direction from Inverness. 



9. That all the observers who heard the noise agree in stating 

 that it was a ' ' rumbling " sound. 



10. That of the fourteen observers within 38 miles of the 

 source who felt the shock, thirteen of them mention having 

 heard the ruilibling noise, and none of the other observers in 

 Scotland mention noise as an accompaniment of the earthquake, 

 and hence that the noise was confined chiefly, if not entirely, to 

 places situated near the source. 



11. That the stations where the noiie was heard were for the 

 most part situated on hard dense rocks, with little or no soil near 

 them. 



12. That the average duiation of the disturbance was 4^4 

 seconds for observers situated within the sound area. 



13. That of twenty-two lighthouse observers between Cape 

 Wrath and the Mull of Galloway who w ere situated on the older 

 formations (Laurentian, Cambrian, and metamorphosed Lo«er 

 Silurian) eleven felt the shock, whilst of thirteen observers 

 situated on nev\ er rocks it made itself known only to two of 

 them, and that the earthquake was therefore more generally felt 

 on the older rocks of Scotland. 



14. That stations situated near one another and on the same 

 formation did not necessarily both receive the shnck, and that 

 faults and trap dykes did not seem to affect the passage or 

 intensity of the wave in any way. 



15. That the observations of time at Armagh, Belfast, and 

 Omagh show that the shocks at these places were most probably 

 propagated direct from Phladda in Scotland, and that the 

 severity of the shock and the " rumbling " noises heard in and 



around Leterkenny were probably due to a second and local 

 source of disturbance generated by the arrival of the shock from 

 Phladda. 



MA GNETIC DEC LIN A TION ' 

 1. fT is well known that Prof. Rudolph Wolf has endeavoured 

 to render observations of sun-spots made at different times, 

 and by different observers, co.nparable with each other, and has 

 thus formed a list exhibiting approximately the relative sun-spot 

 activity for each year. This list extends back into the seven- 

 teeth century, and is unquestionably of much value. Neverthe- 

 less it must be borne in mind that we possess no sun-spot data 

 sufficiently accurate for a discussion in a complete manner of 

 questions relating to solar periodicity before the time when 

 Schwabe had finally matured his system of solar observations, 

 which was not until the year 1832. 



We have however a much longer series of the diurnal ranges 

 of magnetic declination. Now these are already well known to 

 follow very closely all the variations of sun-spot frequency, being 

 greatest when there are most, and least when there are fewest 

 spots ; and it may even be imagined that such ranges give us a 

 better estimate of true solar activity than that which can be 

 derived from the direct measurement of spotted areas. 



The long-period inequalities of the diurnal range of magnetic 

 declination are thus, we may imagine, precisely those of solar 

 activity, so that to analyse the former is probably equivalent to 

 analysing the latter. 



2. Our method of analysis is not new. The system pursued 

 by us is in fact that which has been pursued by Baxendell, and 



probably other astronomers, with observations of variable stars, 

 and it has already been applied by one of us in a preliminary 

 manner to magnetic declination ranges {Proc. Lit. and Phil. 

 Society, Manchester, February 24, 1880). 



3. The observations at our disposal are tho;e which have been 

 used by Prof. Elias Loomis in his comparison of the mean daily 

 range of the magnetic declination with the extent of the black 

 spots on the surface of the sun {Amcritan Journal of Seience 

 and Arts, vol. 1., No. cxlix.). These observations are recorded as 

 monthly means of diurnal declination range, and we found it 

 necessary to multiply each by a certain factor, firstly, on account 

 of the well-known annual inequality of declination range, and 

 secondly, to bring them all to the standard of the Prague obser- 

 vations. We have applied for this latter purpose precisely the 

 same corrections as those made by Prof Loomis. 



4. The result of an analysis of these observations has been to 

 indicate the existence of three inequalities : two dominant ones 

 with periods of about lo4 and 12 years, and a subsidiary one 

 with a period of about i6| years. By these means we have 

 beeen enabled to reproduce the observed annual values of de- 

 clination range with an average difference of 39". The amount 

 of agreement between the observed and calculated values will be 

 seen from a diagram which accompanies this note. We are 

 however of opinion that the series of observed values at present 

 obtainable is too short to render this analysis a very accurate 

 one. It will certainly not bear carrying back forty or fifty years 

 beyond its starting-point, which was in 1784, and it would be 

 very hazardous to carry it forward any considerable length into 

 the future. We may however mention that our calculations 



indicate a maximum of declination range about 18S4, but not so 

 pronounced a maximum as that of 1871. 



5. During our analysis an observation was made by us which 

 we think worthy of record. 



It is a well-known fact that the so-called eleven-yearly oscil- 

 lations of dechnation range are at certain times large, and at 

 other times small. Thus, for instance, they have beeri large for 

 the last forty years, but they were small about the earlier part of 

 the present century. It is clear to us from an inspection of the 

 o'oservations that a series of large oscillations is accompanied 

 with an exaltation of the base line, or line denoting average 

 efficiency, whUe a series of small oscillations is accompanied 

 with a depression of the same. The result is a long-period curve 

 of the base line, the beat period, so to speak, of the eleven- 

 yearly inequality. 



Now a phenomenon precisely similar occurs in connection 

 with shorter periods. If we take inequalities having a period 

 of three or f.ur months we find that such are alternately well 

 developed or of large range, and badly developed, or of small 

 range ; and that a large range of such is accompanied wiih an 

 exaltation of the base line or line of average efficiency, while a 

 small range is accompanied with a depression of the same. The 

 result is a curve of the base line, of which the period is, roughly 

 speaking, eleven years. May we not therefore ima;jine that the 

 so-called eleven-yearly period, or, to speak more correctly, the 



■ "Note en an Attempt to Analyse the Recorded Diurnal Ranges of 

 Magretic Declinaticn." By Balfnir Stewart, M.A., LL.D., F.R S.. Pro- 

 fessor of Natural Philosophy at the Owens College, and William Dodgson. 

 Read at the Manchester Literary and Philosophical Society, March 8. 



