364 De la Rive on the Aurora Borealis. 
negative result is confirmed by Mr. Macquorn Rankine, who has 
shown that this absence of polarisation is not due to the feeble- 
ness of the light, since this same light viewed after reflection from 
water is found to be polarised by this reflection. The most care- 
ful study and experiment have found no trace of polarisation in 
electric light, whether the discharges be made in the air or ina 
vacuum. his is a new proof of the identity of these two kinds 
of phenomena. 
Finally, we discover in the resemblance between auroral ap- 
pearances and certain clouds, as well as the disturbances of the 
magnetic needle, a further important confirmation of our theory. 
The observations of Dr. Richardson already mentioned, which 
show that the aurora exists at moderate elevations, also indicate 
that it is often connected with the formation of different kinds of 
cirro-stratus clouds. Lieutenant Hood, in speaking of the lumi- 
nous bands or columns of the aurora, says that he is convinced 
that they are carried. by the wind, because they retain exactly 
their relative situation, which is not the case when the luminous 
matter moves in the air by its own direct action. Finally, the 
coexistence of the aurora,with small ice needles in the atmos- 
phere, such as exist in elevated clouds, is shown by Captain — 
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are traversed by electric discharges sufficiently energetic, prov 
ded daylight does not efface the feeble light. som 
‘times be detected in the day: thus Arago establishes most jncon- 
testably that Dr. H. Usher was not deceived in a notice publish 
in volume II. of the Memoirs of the Irish Academy, where he 
describes an aurora seen at mid-day on the 24th of May, 1788. 
