Dec. 8, 1881} 
NATURE 
131 
the se¢¢s, it continues to grow on their surface, and if the 
vegetation, which rises two or three metres above the 
water, is burnt, it soon reappears again, reaching a height 
of one metre and more after eight or ten days. The 
thread-like roots of the grass form a kind of rough felt, 
in which palms are sometimes inclosed, whilst masses 
of ooze fill up the interstices between the roots, and form 
thus true dams across the river. When the barrier has 
not yet reached a great size, it might be occasionally de- 
stroyed by the pressure of water accumulated above it ; 
but, as several barriers are formed at the same time at 
various places, the upper one being destroyed, its débris 
is brought to the lower one, and accumulates above it, or 
presses beneath it. The elasticity and tenacity of these 
dams is so great that a steamer attempting to enter it is 
soon repelled by the elasticity of the grass, while men and 
even cattle can easily stay on the floating grass without 
danger. The river is thus soon transformed into a marsh 
covered with a mighty grass vegetation, and the water 
expands to the neighbouring maze, seeking its way 
through many new channels. It is obvious that those 
parts of the river where its bed is more definite are espe- 
cially liable to be obstructed by grass islands which are 
formed in those parts of it where there is no definite 
frontier between running water and marsh. As to the 
appearance of sets, M. Marno is of opinion that they 
have become more frequent during these last years; he 
sees in their frequency a proof of the gradual levelling of 
the whole region by fluviatile deposits and of the general 
transformation of the whole of the region into marshes. 
The high floods of 1878 have largely contributed to the 
formation of numerous floating grass islands and to the 
formation of several large barriers across the river. Of 
course any hydrographical works for preventing the 
formation of grass obstructions would be very difficult 
now, Owing to the scarcity of population ; but the planting 
of papyrus palms along the banks of the chief channel 
would be most useful, as it would prevent the floating 
grass islands formed in the azje from entering into the 
main channel of the Nile. 
THE WEATHER OF NOVEMBER, 1881 
HE weather of November last has been in many 
respects so unusual as to call for a brief record of its 
chief characteristics. For thirteen months previously the 
immense majority of the depression-centres, or centres of 
the storms which swept across North-Western Europe, 
passed to the southward of the northern half of the 
British Islands, and many of them wholly to the south of 
these islands, with the inevitable result of unseasonably 
cold weather to the north of these storm tracts. But early 
in November an important change set in, and up to the 
time of going to press the change has been an enduring 
one, viz. the storms of North-Western Europe have 
swept eastward along tracts wholly to the westward and 
northward of the British Islands, with the necessary re- 
sult of a temperature very greatly in excess of the average 
of the month. 
From Buchan’s isobars for the month we see that the 
mean increase of atmospheric pressure from the Butt of 
Lewis to Valentia, in the south-west of Ireland, is about 
o°100 inch; but in November last the increase amounted 
to 0°348 inch, the means of these places being respec- 
tively 29°391 inches and 29°739 inches. The increase 
from the Butt of Lewes to Dover was still greater, 
amounting to no less than 0’605 inch, instead of o'150 
inch, the normal difference. It is premature to state the 
locus of the centre of this extraordinary barometric de- 
pression till fuller observations have been received ; in 
_the meantime, however, a position in the Atlantic, a 
little to westward of the Hebrides, may be provisionally 
assumed as the centre with but a small limit of error. 
The most important result of this abnormal diminution 
of atmospheric pressure in the north-west, and rapid 
increase southward, has been a prevalence of winds from 
the Atlantic, characterised by a force and a persistency 
quite unprecedented during the last quarter of a century, 
with a distribution of temperature and rainfall over the 
British Islands very remarkable and in some respects 
strikingly abnormal. As these winds from the Atlantic 
swept across and reached the east of Scotland, their 
direction took a more southerly, and in the north a more 
south-easterly course. 
Everywhere the temperature was abnormally in excess 
—-the smallest excess, about 3°5, being on the coast in 
the north ; and the largest excess being in the interior, 
as happens with high temperatures at this time of the 
year, since in such circumstances the cooling through 
terrestrial radiation is relatively much less than usual in 
strictly inland situations. The greatest excess would 
appear to have occurred in the higher parts of the valleys 
of the Thames and Trent in England, and of the Clyde 
and Tweed in Scotland, where it reached, or closely ap- 
proached to, 6°°5 above the means of November for the 
respective districts. In London and Edinburgh the 
excess was 6°'0, 
Oitcomparing this excess for Edinburgh with the ob- 
servations made in that division of the British Islands 
during the past 118 years, or since 1764, the mean tem- 
perature of November, 1881, is absolutely the warmest 
on record, the nearest to it being an excess of 5°°5 in 
1818, and 5°°2 in 1792 and 1847. As regards London, 
the temperature of November 1818 and 1852 somewhat 
exceeded that of 1881, the former of these years being 
also unusually warm in Edinburgh, whereas there No- 
vember, 1852, was colder than the average. 
The distribution of the rainfall was strikingly unequal 
in North britain, or where the prevailing winds curved 
round more towards a southerly and south-easterly direc- 
tion. On the high ground sloping up on both sides to the 
Lead and Lowther Hills the rainfall at many places con- 
siderably exceeded double the average of the month. On 
the other hand to the north of the Cheviots and Lammer- 
moors the rainfall was under the average, the amount in 
East Lothian being less than half the average. Crossing 
the Firth of Forth, we meet an extensive tract reaching 
as far as the high grounds of the Grampians, where the 
rainfall was excessive, amounting in West Perthshire and 
Upper Dee to more than double the average. Again, 
beyond the Grampians, and including the whole of the 
North of Scotland, northward and westward to the 
extreme north of the Lewis, the fall was less than the 
average, the amount on the south shores of the Moray 
Firth being only half the average. It is worthy of remark 
that this distribution of the rainfall is precisely the oppo- 
site of what occurs with weather very similar, but with 
the single difference of the south and south-east winds 
being replaced by north and north-east winds, in which 
the foreshores of the Forth, Moray, and Pentland Firths 
facing the north are deluged with rains. In the east of 
England the rainfall was, generally speaking, light, but it 
was above the average in Ireland, and in a less degree in 
the west of England. 
Out in the Atlantic, along the great routes of traffic to 
New York, the month would appear to have been charac- 
terised by an almost unbroken succession of storms, 
several of which, if judged by their destructive effects on 
even well-appointed sailing-vessels and steamers, were 
memorably great storms. The storm which reached the 
west on the 2!Ist will be long remembered for the furious 
winds and extraordinarily high and destructive tides 
which accompanied it; and the storm of Sunday, the 
27th, for its most disastrous effects, particularly in the 
south, and for the unprecedentedly low readings of the 
barometer in the north-west and north of Scotland, where, 
over a wide area and for a considerable time, atmo- 
