The Aurora . 
[June, 
380 
On the general theory of the Aurora Mr. Capron is not 
very definite. He recounts a large number of experiments 
upon the adtion of powerful magnets upon the luminous 
discharges in vacuous tubes, undertaken with a view to elicit 
evidence on this question. His results show no important 
advance upon the researches of Pliicker and De la Rive, 
and require, indeed, to be repeated by the light of the still 
more recent researches of Mr. Crookes, with which our 
readers are familiar. . 
Substantially Mr. Capron adopts Franklin s view, that 
the light of the Aurora is due to an eledtric discharge 
through the moistened air, the ele&ricity concerned in the 
phenomenon being the produdt of evaporation in the region 
between the tropics, and passing at a great elevation into 
the Polar regions. This theory, it will be seen, almost 
precisely anticipates the theory of the Aurora put forward 
last autumn by Prof. Rowland. The theory of Lemstrom, 
and an account of the instrument devised by that physicist 
to illustrate the production of light by eledtric dischaiges as 
they pass into a stratum of rarefied air, are also given. 
Lemstrom takes Dalton’s view, that the earth s magnetism 
adts in directing the position of the auroral discharge, but 
holds that the essential and unique cause of the formation 
of the [light is the quiet discharge of positive eledtncity 
from the upper atmosphere to the earth— a discharge which 
has its counterpart in the lightning sparks of equatorial and 
temperate latitudes, but which differs from these simply by 
reason of the better conducting power of the colder and 
moister streams of air at the Poles. 
Altogether the volume is, in spite of the drawback 
named above, a most valuable and admirable work, and 
we congratulate the author most sincerely on its timely 
appearance. 
