142 ANNUAL OF SCIENTIFIC DISCOVERY. 



the Polar Lights themselves, especially those which are susceptible of precise 

 measurement and instrumental observation, conspire to verify Faraday's sug- 

 gestion as to their immediate nature and cause. That they are truly electri- 

 cal in their nature, an inference rendered so probable by their obvious phe- 

 nomena, Mr. Brayley considered to be proved by their (electro-magnetic 

 inductive) effects on the magnetic elements; nothing hitherto known having 

 the power of producing such effects but magnetism itself, and electricity, 

 while no phenomena of the former are luminous, there is no magnetic 

 light; and the absence of atmospheric electricity during the display of the 

 aurora, paradoxical as it may seem, is a necessary consequence, the electric- 

 ity being absorbed, as it were, by its conversion into the correlate magnetism ; 

 or, in other words, ceasing to be statically manifested while being dynamically 

 exerted. Some experimental illustrations of the electrical nature of the 

 Polar Lights were then exhibited, in which the luminous disruptive discharge 

 was taken in exhausted tubes, that is, in excessively rare media resembling 

 in their attenuation the atmosphere itself at the elevations where the Aurora 

 occurs; one of the tubes, prepared by M. Gassiot, showing the stratified dis- 

 charge (originally obtained by Mr. Grove), recently cited by Humboldt in 

 evidence that the dark space in the Aurora may be real, and not merely the 

 effect of contrast. The source of the electricity in these experiments being 

 the apparatus termed the Ruhmkorff coil, the close accordance between them 

 and the natural phenomenon was pointed out, in the fact that the electricity 

 was obtained by a process of magneto-electric induction, exactly analogous, 

 on the small scale, to the natural process to which, operating in the globe 

 itself, Faraday has referred the electricity manifested in the Polar Lights. 

 The actual influence of the Aurora on the magnetic elements Avas exemplified 

 by three photographs from the self-registering apparatus at the Kcw Obser- 

 vatory, on which the vertical, the horizontal, and the total-force magnetome- 

 ters, respectively, had recorded the disturbances produced in them by the 

 Aurora of December 3, 18-38. The facts establishing the participation of the 

 Polar Light in the great law of solar periodicity which it had been the object 

 of the lecturer thus generally to explain, were then briefly stated; and the 

 conclusion was deduced, that the relation of the periodicity to the electrical 

 causation of the Polar Light is simply this, that the magnetic action of the 

 Sun periodically affects the terrestrial magnetism, which being converted into 

 electricity by the earth's rotation and moving conductors, agreeably to the 

 theory maintained, exhibits the period in the polar discharges of that 

 electricity. 



MAGNETIC DECLINATION. 



From a table published by Encke, in the Memoirs of the Berlin Academy, 

 it appears that, in the fifteen years between 1839 and 1854, the magnetic 

 " declination," or the westerly deviation of the magnetic north from the true 

 noi-th, has diminished 1 49F; the "variation" has, therefore, been at the 

 mean rate of 7| minutes per annum; but it has been a little greater in the 

 second half of the term than the first. The declination at Berlin in 18-34 was 

 14 50' 5Q". 



At a meeting of the Royal Belgian Academy, a letter was read from Han- 

 stccn to the secretary, M. Quetelet, stating that with one of Gambey's 

 needles, aided by careful manipulation, he is able to take the precise dip 

 within at least half a minute. From observations made in four summer 

 months with a dipping-needle and unifilar and bifJar horizontal needles, he 



