4 Transactions. — Miscellaneous. 



niaximnm of 24° 41' was reaolied. Since then it has gradually diminished : it 

 was 22° 30' in 1850; in 1866, 20° 25' ; and is probably at the present date 

 some few minutes under 20° W. 



The annual variation at Paris and London is greatest about the latter end 

 of March in each year, diminishing from that time to the latter end of June, 

 and increasing again during the following nine months. It does not exceed 

 from 15' to 18', and it varies somewhat at different epochs. 



The diurnal variation differs according to the time of year and place of 

 observation, the mean daily range in London being about 9 '3 minutes; in 

 Paris, about 11*5 minutes. The amplitude of the daily variations is greatest 

 from April to September. 



The declination is accidentally disturbed in its daily variations by many 

 causes, such as earthquakes, volcanic eruptions, and the aurora borealis and 

 aurora australis. In Ganot's " Elements of Physics" it is said that "the effect 

 of the aurora is felt at great distances. Auroras which are only visible in the 

 north of Europe act on the needle even in these latitudes (that is, of London 

 and Paris), where accidental variations of 20' have been observed. In polar 

 regions the needle frequently oscillates several degrees ; its irregularity on the 

 day before, the aurora borealis is a presage of the occurrence of this 

 phenomenon." Although there is little doubt that the declination of the 

 needle is generally affected to a greater or less extent before and during the 

 aurora, yet this is not invariably the case, as the following extract from 

 Captain Parry's narrative of his third voyage for the discovery of a north-west 

 passage will show. The extract is so short and so pertinent to the subject 

 under consideration that I make no apology for introducing it. SjDeaking of 

 the aurora borealis, he says : — 



" About midnight on the 27th of January, 1825, this phenomenon broke 

 out in a single compact mass of brilliant yellow light, situated about a south-east 

 beai'ing, and appearing only a short distance above the land. This mass of 

 light, notwithstanding its general continuity, sometimes appeared to be 

 evidently composed of numerous pencils of rays, compressed, as it were, 

 laterally into one, its limits, both to the right and left, being well defined and 

 nearly vertical. The light, though very bright at all times, vai-ied almost 

 constantly in intensity, and this had the appearance (not an uncommon one in 

 the aurora) of being produced by one volume of light overlying another, just 

 as we see the darkness and density of smoke increased by cloud rolling over 

 cloud. While Lieutenants Sherer and Ross and myself were admiring the 

 extx-eme beauty of this phenomenon from the obsei-vatory, we all simultaneously 

 littered an exclamation of surprise at seeing a bright I'ay of the aurora shoot 

 suddenly downward from the general mass of light, and between us and the 

 land, which was then distant only three thousand yards. Had I witnessed 



