NATURAL PHILOSOPHY. 115 



out of the tubulure. The bar is hermetically sealed in the tubulure, 

 and covered throughout its length, except at its two ends, with an 

 isolating and thick layer of wax. A copper ring surrounds the bar 

 above the isolating surface in its internal part the nearest to the side 

 of the globe ; from this ring proceeds a conducting rod, which, care- 

 fully isolated, traverses the same tubulure as the iron bar, but without 

 communicating with it, and terminates externally in a knob. When, 

 by means of a stopcock adjusted to the second tubulure of the globe, 

 the air in it is rarefied, the knob is made to communicate with one of 

 the conductors of an electric machine, and the external extremity of 

 the iron bar with the other, so that the two electricities unite in the 

 interior of the globe, forming between the internal extremity of the 

 iron bar and the copper ring which is at its base a more or less regu- 

 lar fascicle of light. But if the external extremity of the iron bar is 

 placed in contact with one of the poles of a strong electro-magnet, 

 taking good care to preserve the isolation, the electric light takes a 

 very different aspect. Instead of issuing, as before, from the different 

 points of the surface of the terminal part of the iron bar, it is emitted 

 only from the points which form the contour of this part, so as to con- 

 stitute a continuous luminous ring. This is not all ; this ring and the 

 luminous jets which emanate from it have a continuous movement of 

 rotation around the magnetized bar, now in one direction, now in 

 another, according to the electric discharges and the direction of the 

 magnetization. Lastly, more brilliant jets appear to issue from this 

 luminous circumference, without being confounded with those which 

 terminate on the ring and from the fascicle. As soon as the mag- 

 netization ceases, the luminous phenomena becomes again what it was 

 previously, and what it is generally in the experiment known as the 

 electrical egg. 



This experiment appears to account very satisfactorily for what 

 passes in the phenomena of the aurora borealis ; in fact, the light 

 which results from the union of the two electricities in the part of the 

 atmosphere w-hich covers the polar regions, instead of remaining 

 vaguely distributed, is carried by the action of the terrestrial magnetism 

 round the magnetic pole of the globe, whence it seems to rise in a re- 

 volving column, of which it is the base. We thus understand why 

 the magnetic pole is always the apparent centre whence issues the light 

 constituting the aurora borealis, or towards which it appears to con- 

 verge. Stttimari's Journal, May, 1850. 



LAWS AND ORIGIN OF THE AURORA BOREALIS. 



PROFESSOR OLMSTED presented to the American Association, at New 

 Haven, a paper on the aurora borealis, in which he endeavoured to 

 show that we have just passed through an extraordinary period of 

 auroras, which he called a " visitation," which commenced in 1827, 

 and closed in 1848. He then proceeded to lay down the laws of the 

 phenomenon, as deduced from a large number of facts. They are as 

 follows : 



1. That the aurora of the first class usually commences near the end 



