192 
NAITORE 
[APRIL 23, 1914 
quires a pressure applied for some time to make 
them stick together. The resulting forms taken 
by accumulations near obstacles differ consider- 
ably in consequence. 
One of the most interesting observations in the 
book relates to the natural sifting which sand 
undergoes whilst being blown hither and thither 
by the wind, One sample of desert sand was 
passed by the author through a series of wire 
gauze sieves of graduated mesh; a single sieve 
with a 1/48 in. mesh retained 94 
per cent. of the total, the sieve 
above with a 1/24 in. mesh stop- 
ping 2 per cent. and the one below 
with a 1/96 in. mesh stopping 4 
per -cent. Practically, therefore, 
94 per cent. of the sand grains had 
linear dimensions of between 0°02 
and o’or in. 
It would have been of interest 
if this sorting test had been carried 
further, for several phenomena of 
sand, notably “singing sand,” and 
also the extraordinary roar which 
is sometimes heard when a slip 
occurs in a slope of blown sand, 
must depend on the uniformity of 
the size of the grains. Darwin in 
his voyage of the Beagle refers in 
chapter xvi. to a hill in Chile 
known as “El Bramante,”- on 
account of the roaring sound pro- 
-duced by the slipping of sand, and 
also states that the same circum- 
stances are described in detail by 
Leetzen and Ehrenberg as_ the 
cause of the sounds which have 
been heard by many travellers on 
Mount Sinai. I have had a de- 
scription from a friend who, with 
a party, was descending a slope 
of blown sand drifted against a 
cliff in the Nile valley. So far as 
could be seen, only a small surface 
flow of sand started by their foot- 
steps appeared to be in motion, 
but the noise gradually increased 
to a loud roar, and the whole mass 
of the drift seemed to vibrate. 
This implies that each grain was 
doing the same thing at the same 
time for a considerable depth, 
which could scarcely happen were 
there not a fairly close uniformity 
in their size. 
How the sorting is carried out by the wind 
does not clearly appear. Dr. Cornish’s explana- 
tion is that the predominant size of grain is 
reached when mutual attrition ceases. If this is 
correct, it might be possible to determine the size 
in terms of the hardness of the material and the 
mean square velocity of impact. There is no 
doubt a definite size for which the whole work 
of impact could be taken up by elasticity and 
without rupture. The whole question, however, 
NO) 2321p VOL 3) 
BUIGae 
1. —A nine-foot snow-mushroom seen from below. 
of the way in which dust is raised by the wind 
is rather obscure. Presumably the wind in con- 
tact with the ground must move parallel to its 
surface, and it seems probable that particles drift- 
ing along the surface can only be raised above 
it by impact more or less oblique with others 
which are stationary or moving with a different 
velocity. Once they are lifted into the eddying 
current their further distribution does not present 
the same sort of difficulty. 
Poel 
- ves 
From ‘‘ Waves of Sand and Snow.’ 
Any structure which shows a “period” always 
presents interesting problems, but the periods 
and wave-lengths which Dr. Cornish deals with 
must not be confused with those belonging to 
stable systems, such as water waves, etc. The 
latter are definite in the same way and for the 
same reason as the period of a pendulum. 
The sand waves are products of instability, and 
in all quasi-periodic structures which originate 
in this way the amplitude and wave- length are 
