DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 581 
and Hobarton. It will be remarked, however, that the ratios for the European 
Continent are markedly greater than for the other parts of the earth.* 
It appears, also, from the results for Trevandrum, that the ratio is not quite 
constant, but is diminishing gradually. This fact, if followed through succes- 
sive periods, will probably lead to the knowledge of a periodic variation. I 
believe Trevandrum is a station particularly suited for the determination of 
laws depending on small differences, since there the irregular effect of magnetic 
disturbance is much less felt than in high latitudes. 
35. Ratios of the Ranges derived from the Movements between different Hours. 
—If the diurnal movement of the magnetic needle follows the same law, at the 
same place, in years of maximum as in years of minimum disturbance, we may 
conclude that the ratio of the movements between any two hours for these 
years will be constant. The following are the ratios of the yearly mean move- 
ments for 1848°25 and 1843°5, between different hours and the hour of 
maximum at Hobarton :— 
From all hours. 20 to 2h 21 to Qh 
11'-69 10’-31 11:25 
eee ee c ge ene = “48 ee nl 
7°68 nee 6°97 : z 7°43 
22h to 2h 235 to 2h 04 to 2h 
9'°32 al 4’-49 
Sal cao Si ‘40 : pee = 1°59 ae SIGNS 
6 64 : 2 4 86 2°71 
* This fact is confirmed by the observations made at Lisbon, at 8 a.m. and 2 p.m., by Mr CapgE.to, 
These give, for the middle of the years, of maximum and minimum— 
1859°5 SSS 10r 54 


— = 1°71 
1867°5 virranene oi i) 
1870°5 : 10°83 ae 1°78 
1875°5 6 ‘09 
(“ Nature,” April 1876, p. 448.) The first ratio is almost the same as that obtained from the Munich 
observations for the same hours and years. We may conclude, also, that had the means for the 
exact epochs of maximum and minimum been obtained, the ratios would have been still greater. On 
the other hand, the Milan observations do not agree with those at Munich and Trevandrum in 
showing a nearly constant ratio for different periods; and the ratios for Milan differ considerably from 
those for Munich and Lisbon. Thus, the mean ranges for the years 1859°5 and 1866°5, give the 
following ratios :— 
Munich. Milan. Lisbon. 
1859°5 11"16 10’00 10°54 
ees see ce ILA fo See SS SSS I, 
1866°5 ; 6°50 ‘ 4°24 6°19 
The Milan ratios for the true maxima and minima also vary from 
1848°3 11°64 1870:8 _»uh2/-00 
1844-9 CulT 1859°3 4-01 
We can explain these great differences of results from two places so near as Munich and Milan, 
while stations so far apart as Munich and Lisbon agree so well, only on the supposition of some 
remarkable Jucal cause. We have here another instance of the necessity of having two instruments at 
the same station to control each other, and to decide what results are instrumental and what not. 
The above quantities for Milan are taken from Dr Wor’ Astron. Mitth. xxxvili. p. 382 (July 
1875), where I have found, since this paper was written, that the Zurich astronomer has adopted the 
method of yearly means corresponding to each month, 
1°72 , to 299, 


