May 8, 1902] 
“results of observation,” judging from an interesting article by 
Prof. William H. Pickering in the May number of the Century 
Magazine. 
the last few years made numerous excellent observations on 
the planet Mars, and they have greatly increased our knowledge 
by accurately observing the surface markings and suggesting very 
plausible explanations of the phenomena observed. Such work 
was rendered possible by erecting an observatory in a locality 
where observing conditions were as near perfect -as_ possible. 
Prof. Pickering has more recently turned his attention to an 
examination of the lunar surface, and the first results of this 
work have led him to some very definite and striking conclusions. 
The first of these is that there seems to be strong, if not 
fairly conclusive, evidence in favour of the idea that volcanic 
activity has not yet entirely ceased, and he quotes several 
Messrs. Pickering and Percival Lowell have during | 
NATURE 
instances in which small craters have disappeared while others — 
have sprung up in different regions. The second, and perhaps 
more startling, announcement is that there is snow on the moon. 
He has observed that many craterlets are lined with a white 
substance which becomes very brilliant when illuminated by the 
sun, and a similar substance is found on the larger lunar craters 
and a few of the higher mountain peaks. The curious behaviour of 
these patches under different angles of illumination and their 
change of form have led him to suggest that an irregularly 
varying distribution of hoar frost may-have something to do with 
the changes observed. The third remarkable deduction refers 
to the observations of ‘‘ variable spots,” which appear to be 
restricted between latitudes 55° north and 60° south ; these spots 
are always associated with small craterlets or deep narrow clefts, 
and are often symmetrically arranged around the former. The 
alterations which these undergo have led him to seek the cause 
in the change in the nature of the reflecting surface, and the 
most simple explanation according to him is found in assuming 
that it is organic life resembling vegetation, but not necessarily 
identical with it. The new selenography consists, therefore, as 
Prof. Pickering remarks, ‘‘not in mere mapping of cold dead 
rocks and isolated craters, but in a study of the daily alterations 
that take place in small selected regions, where we find real, 
living changes, changes that cannot be explained by shifting 
shadows or varying librations of the lunar surface.” Prof. 
Pickering illustrates his article with numerous excellent and 
instructive drawings and photographs of portions of the lunar 
surface, and these give the reader a good idea of the changes 
referred to in the text. 
7 
DUST-FALLS AND THEIR ORIGINS. 
FALLS of dust on a large scale are of rare occurrence, but 
one very often hears that in the south of Europe at such 
and such a place rain had fallen and had brought with it, and 
deposited on the ground, fine red or yellow dust. 
April 24, 1897, a south wind carried to southern Italy a 
great quantity of dust which was supposed to be of African 
origin. 
Perhaps the most well-known instance of a fall ona large 
scale was that which occurred in May and August in the year 
1883, when an enormous quantity of dust was hurled into 
the air during the Krakatoa eruption, and fell and was collected 
at various distances, the greatest being more than 1100 miles 
from the seat of the disturbance. The tremendous height to 
which the finer particles of dust were thrown, coupled with the 
movement of the air at this great distance from the earth’s 
surface, were responsible for the magnificent coloured sunsets 
which were observed nearly all overthe world. The volume? in 
which all these observations were collected is undoubtedly one 
of the most complete records of a ‘‘ fall of dust” that has been 
published. 
The large number of meteorological stations situated over the 
greater portion of the civilised world give us now greater chances 
for recording and tracing the paths of these falls of dust, 
whether they reach the earth’s surface with or without the aid 
of rain. Fortunately, the tracks of the great dust storm of 
March 9-12 of last year and that of the minor storm of March 
1 “Der grosse Staubfall von 9 bis 12 Marz, 1901, in Nordafrica, Sud- 
und Mitteleuropa.” Von G. Hellmann und W. Meinardus. Abhandlungen 
des Konigtich Preussischen Meteorologischen Institut, Bd. ii. No. 1. 
(Berlin: A. Asher and Co., rgor.) 
2 ** Report of the Krakatoa Committee of the Royal Society.’’ (London : 
Trubner and Co., 1838.) 
NO. 1697, VOL. 66] 
4 
19-21 of the same year were restricted to such regions as these, 
passing over the coast of northern Africa and reaching Sicily, 
Italy, Austro-Hungary, Prus-ia, part of Russia, Denmark and 
even the British Isles. 
In the volume before us, Profs. Hellmann and Meinardus 
have brought together all the information that could be col- 
lected by means of the distribution of circulars and communi- 
cations with all meteorological stations, and discussed them in a 
very thorough and able manner, presenting us with a complete 
story, describing the locality from which the dust came, the 
means, direction and mode of transport, and finally the places 
over which it was deposited. The arrangement of the discus- 
sion is as follows :—The distribution of the dust over the land 
surface is first described, accompanied by the original accounts 
of the phenomenon as observed, a list of all places where the fall 
was recorded, and a map showing the general distribution. The 
meteorological conditions from March 9-12 are next dealt 
with, giving full details of the general atmospheric disturbances 
over the whole of Europe and North Africa, with numerous 
maps. The authors then give the individual reports on all the 
microscopic and chemical analyses of the dust from various 
localities, concluding with a brief account of the second fall of 
dust from March 19-21 and a general summary of the main 
results to which they have been led. 
In these chapters the discussion of the facts collected has led 
the investigators to form a very concrete survey of the whole 
| phenomenon, tracing the origin of the dust to dust-storms that 
Thus on | 
occurred on March 8, 9 and 1o in the desert El Erg, situated 
in the southern part of Algeria, and which carried the dust and 
transported it northward. 
This dust, as is here pointed out, began to fall at Algiers and 
Tunis in the dry state on the night of the 9th. The subsequent 
falls gradually took place northwards, first Sicily, then Italy, 
the Alps, Austro-Hungary, Germany, Denmark and European 
Russia, practically in the order named, coming in for their 
share. In Sicily and Italy the dust was noticed to have fallen 
even without the aid of rain, but in the other countries it was 
only detected during and after showers. 
Not only did the dust-fall occur in these countries in the sequence 
mentioned, but the quantity that fell became gradually less the 
more north the places were situated, and the fineness of the 
dust, as shown by the analyses, increased at the same time. 
All these facts, as the authors indicate, are strong arguments in 
favour of the progress of the dust deposition from south to 
north, and the very minute and careful examination of the 
meteorological conditions stated here, showing a depression 
moving from south to north, endorse this point of view. There 
is little doubt, therefore, that the locality from which the dust 
originated was situated somewhere south of the northern shore 
of the African continent. 
It is interesting to notice that the dust was not distributed 
homogeneously over the land areas, but in patches and streaks, 
some places, such as, for instance, the greater part of south 
Germany and Russian Poland, being entirely free from it, 
_ while others, such as the southern side of the eastern Alps and 
Holstein, being specially dense. The unequal distribution and 
different values for the rate of movement of the dust cloud 
seem to be adequately explained by the variable velocity of the 
air currents and the changing position of the barometric 
depression. 
The investigation suggests that the dust was carried by a 
large mass of air which moved with great velocity from northern 
Africa to the north of Europe, and that this mass of air, 
cyclonic in nature, was fed on its western side by air currents 
from the north and on its eastern side by southerly currents ; 
this accounts for the observed facts that the fall of dust was 
chiefly limited to the eastern portion of the depression. 
As regards the total amount of dust that fell to the surface, 
rough estimates indicated that the weight of it would amount to 
about 1,800,000 tons, two-thirds of which were deposited to 
| the south of the Alps. 
The authors have shown that the most probable origin of the 
dust was the region to the south of Algeria, so that an examin- 
ation of the dust that fell in Europe and elsewhere should 
consist of similar components as those that form the dust of this 
region. Nearly all the mineralogical, microscopic and chemical 
analyses point out that the dust is neither volcanic nor cosmic, 
but simply such as is found on the African continent. From 
exactly which part of the continent it came is evidently not 
certain, for some mineralogists suggested that the dust consisted 
