a 
AVENA. 
cribed it to the reflection of the sun and moon 
by the clouds of snow and needles of ice, which 
are constantly floating in the atmosphere of the 
frigid zones. Mairan supposed it to proceed from 
the atmosphere of the sun. Bailly ascribed it 
to magnetism, and its remarkable influence on 
the needle has been generally observed. Frank- 
lin attributed it to electricity. Biot, who was sent 
to the Shetland islands, in 1817, by the French 
academy of sciences, to determine the length of 
the pendulum vibrating seconds, had an oppor- 
tunity, August 27th of the same year, of observing 
the aurora borealis, in all its splendour, at the 
island of Unst. On this occasion, he ascribed to 
the phenomenon a volcanic origin, and his rea- 
soning is given at length in the ‘ Journal des 
Savans’ for 1820. His description of this wonder- 
ful phenomenon is to be found in Biot’s ‘ Précis 
Elémentaire de Physique,’ 3d edit., Paris, 1824, 
vol. ii. p. 99, et seq. An ingenious hint of Kast- 
| ner, advanced in the sixth edition of Gren’s 
‘ Physik,’ Halle, 1820, is deserving of attention. 
| He considers polar lights as the electricity of the 
| earth rising periodically to the poles. 
_ teresting observations on this appearance were 
' communicated by Richardson and Hood, in the 
| appendix to Franklin’s ‘ Narrative of a Journey 
_to the Shores of the Polar Sea, in 1819, &c. 
Some in- 
London, 1823, 1824. Notwithstanding the atten- 
| tion which has been paid to the phenomena of 
the aurora borealis, and the various hypotheses 
which have been imagined to explain them, it 
will be found that there is a want of information 
on the points which are most necessary as bases 
of induction; and the British Association have 
therefore been induced to appoint a committee 
in the express view of directing observers to the 
really important features of this meteor, and of 
obtaining, by a system of contemporaneous ob- 
servation, data which experience shows cannot 
be derived from insulated exertion. 
The following are the most important points 
which demand the attention of observers :— 
1. The elevation of the auroral arches ‘and 
streamers above the surface of the earth. 
2. The determination of the question whether 
the auroral exhibition is accompanied by sound. 
3. The existence of recurring periods of fre- 
quency and brilliancy in the aurora. 
4, The influence of arches, streamers, and other 
auroral phenomena upon the magnetic needle. 
AUTUMN. See Szasons. 
AVENA,—popularly Oat-grass. A large and 
important genus of grasses, forming the type of 
the suborder Avenacez or oatlike grasses. This 
suborder comprises the genera avena, danthonia, 
trisetum, tricheta, deschampsia, ammophila, cory- 
nephorus, arrhenatherum, holcus, hierocloe, aira, 
peribalia, melica, atropsis, schismus, triodia, 
phragmites, arundo, and arundinaria. The genus 
Avena, as formerly constituted, and as still un- 
derstood by many botanists, includes some spe- 
cies which have recently been assigned to five or 
295 
six of these genera: yet, independently of these 
species, it comprises about forty known species, 
seventeen of which are either indigenous in 
Great Britain, or have been introduced from 
foreign countries. 
venience, treat the genus as still including the 
few species which have been assigned to other 
genera; and, thus understood, the portion of it 
which exists in Britain comprises five species 
which are cultivated as grain, three species 
which are valuable as forage-grasses, and several 
species which challenge attention only as weeds. 
The short oat, Avena brevis, is an annual cereal 
grass, Introduced from Germany in 1804. It 
grows to the height of only about 9 inches; and 
its seeds are small but numerous. It is culti- 
vated, for the sake of its seeds, in the central 
uplands of France, and in other districts —The 
bristle-pointed oat, Avena strigosa, but now called 
Danthonia strigosa, grows wild, as an annual 
weed, in the hedges of Great Britain; yet is 
sometimes cultivated for its seeds in the remoter 
districts of the Scottish Highlands. A variety 
of this species is known to British agriculturists, 
under the name of the Argyleshire small oat.— 
The eastern or Tartarian oat, Avena orientalis, 
is an annual grass, grows to the height of three 
feet, and was introduced to Great Britain in | 
1798. It is distinguished from the common oat 
by its panicle being contracted and nodding to 
In its ill-improved or comparatively wild | 
a side. 
state, it is awned and blackish in colour; but it 
° ° i 
greatly improves, and becomes awnless and white, 
by cultivation in a good soil. It is cultivated as 
a cereal grass, partially in Britain, and largely | 
Three | 
in some districts of continental Europe. 
varieties of this species are known to British 
agriculturists, under the names of black Tar-_| 
tarian oat, common white Tartarian oat, and 
early white Tartarian oat.—The naked oat, Avena | 
nuda, is distinguished from the other cultivated 
species, by having the corolla detached from the 
seed, in the same manner as wheat and naked 
barley ; and, on this account, it is often popularly 
called pilcorn or peelcorn. It has been culti- 
vated from time immemorial in Europe; it is 
mentioned by our earliest writers; and it was, 
at one time, in general cultivation throughout 
Scotland and some parts of England. A variety 
of this species is known to some farmers under 
the name of the small naked oat—The common 
oat, Avena sativa, is incomparably the most im- 
portant species, and it has sported itself into 
numerous valuable varieties, which constitute 
emphatically the oats of agriculture. Two per- 
manent varieties, the blackseeded and the white- 
seeded, or melanosperma and leucosperma, are re- 
cognised by botanists; and about forty distinct 
and rather conspicuous varieties are known to 
the most intelligent class of British agriculturists, 
and have been classified into the three divisions 
of thin-skinned white oats, large thick-skinned 
white oats, and dark-coloured oats. The thin- 
We shall, for the sake of con-. 
