220 
POPULAR SCIENCE REVIEW. 
displays liave been witnessed, especially in 1859. Besides this 
secular variation, there seems to be also a mensual variation. 
The following table, derived from a large number of recorded 
displays, will illustrate this point : — 
PER CEKT. 
January 
February 
March - 
April 
May 
June 
July 
August 
September 
October 
November 
December 
7-0 
94 
13'4 
9'6 
5'7 
2-0 
2'7 
6-7 
12-4 
15-4 
8'8 
6-9 
lOO'O 
Referring the above results to the four quarters of the year, 
we get the following table. The paucity of Aurorae during tlie 
summer months is doubtless partially to be ascribed to the 
shortness of the nights : — 
Spring (March, April, May) 
Summer (June, July, August) 
Autumn (September, October, November) 
Winter (December, January, February) 
PER CENT. 
28-8 
11-4 
36-5 
233 
100-0 
There seems also to be a diurnal periodicity. Bravais 
remarks that auroral displays happen most frequently about 
10 p.m., and rarely after 4 a.m. 
Many observers have thought that they have heard sounds 
emitted by Aurorae. Possibly this is referred to by Virgil, 
who, after describing the prodigies at Caesar’s death, says : — 
<! Arnioi-um sonitum toto Germania coelo 
Audiit.”* 
Much uncertainty exists on this point, as the evidence is very 
conflicting. 
The influence of the Aurora in producing gales of wind 
seems to have been first pointed out by one John Winn, in a 
letter to Dr. Franklin, dated Spithead, August 12, 1772. 
He says : — ■ 
“I believe the observation is new that the Aurora Borealis is 
constantly succeeded by hard southerly or south-west winds, 
* Georgies, lib. i. line 478. 
