4() ON THE RECENT SECULAR PERIOD 



the origin of the vapor, or material itself that forms the basis of the aurora? 

 "What causes the periodicity of thcs£ exhibitions; or why do they occur in certain 

 parts of the twenty-four hours rather than in others? why in certain months of 

 the year more than in other months; and, especially, why do they return in secular 

 periods? Can any explanation be given of their sensible appearances, such as their 

 luminous phenomena, of their remarkable motions, of their definite arrangement in 

 columns, arches, and coronas? Can any reason be assigned why the auroral exhi- 

 bitions take place in the higher latitudes rather than in the equatorial regions, 

 and why they are more intense in the corresponding latitudes on the western than 

 on the eastern continent? A theory of the aurora borealis can hardly be considered 

 as satisfactory unless it can render a full explanation of most of these points, and 

 be not inconsistent with any known facts. Moreover, an explanation that will 

 account for a part only of the facts and render no reason for other facts equally 

 requiring explanation, must be considered as defective and inadequate. Thus a 

 theory which explains merely the luminous appearances, but renders no account of 

 the origin of the aurora itself, must be held as very incomplete, since the origin is 

 the main thing to be accounted for. Nor ought we to lose sight of the distinction 

 between an h}q:>othesis and a theory, an hypothesis being a principle assumed to 

 account for a class of facts, and having no other claims to be considered the true 

 cause except that it explains the facts, while a theory is a deduction from the facts 

 themselves made in accordance with the established laws of nature. We now 

 proceed to consider what causes have been assigned or may be assigned in explana- 

 tion of the aurora borealis. 



In the year 1716 commenced one of those remarkable series of the aurora borealis 

 which we have ventured to denominate secular periods ; and the men of science on 

 the stage at that time viewed the exhibitions with great attention, and a few of 

 them eagerly inquired into their cause. Of these philosophers, the most distin- 

 guished were Halley and Coates in England, and Mairan in France. Ilalley offered 

 the following explanation. He considered the earth as a great magnet, analogous 

 to an artificial spherical magnet, and supposed that a certain subtile matter or 

 eflluvium passing into the pores of the earth near the south pole, and issuing at the 

 north pole, caused both the polarity of the needle and the phenomena of the aurora 

 borealis. 1 This connecting the cause with magnetism was the more remarkable, 

 because the magnetic properties of the aurora itself were then unknown, being first 

 discovered in 1740 by Celsius, and Hiorter, two Swedish philosophers. But with 

 Halley, the idea of such an origin was suggested by his notion that the magnetism 

 of the earth is owing to the circulation of such an "effluvium," which, as he con- 

 ceived, might be so condensed as to form the palpable vapor of the aurora, and so 

 exalted in intensity as to exhibit the luminous appearances connected with the 

 aurora. Since no such effluvium as that supposed is known to exist, it cannot be 

 made the basis of an explanation, and it seems unnecessary to argue further against 

 the hypothesis of Ilalley. 



Roger Coates has left, in the Philosophical Transactions, a very minute description 



1 Phil. Trs., No. 347, or abridged, IV, 138. 



