TRANSACTIONS OF SECTION E. 857 
these races only have, except in one instance, resisted both Arab and European. 
In a note an attempt is made to reconcile the classification given by Herr Engler 
with that adopted in this paper. 
7. Sand Dunes. By Vaucuan Cornisu, I.Sc. 
In the sorting of materials by wind the coarser gravel is left on stony deserts 
or sea-beaches, the sand is heaped up in dune tracts, and the dust (consisting 
largely of friable materials which have been reduced to powder in the dune dis- 
trict itself) forms widely scattered deposits beyond the limits of the dune district. 
Three principal factors operate in dune tracts, viz. (1) the wind ; (2) the eddy in 
the lee of each obstacle ; (3) gravity. The wind drifts the fine and the coarse sand. 
The upward motion of the eddy lifts the fine sand and, co-operating with the wind, 
sends it flying from the crest of the dune. The backward motion of the eddy 
arrests the forward drift of the coarser sand, and thus co-operates with the wind 
to build the permanent structure of the dune. Gravity reduces to the angle of 
rest any slopes which have been forced to a steeper pitch either by wind or eddy ; 
hence in a group of dunes the amplitude cannot be greater than (about) one-third 
cf the wave-length. This limit is most nearly approached when the wind blows 
alternately from opposite quarters, but does not hold in one quarter sufficiently 
long to completely reverse the work of preceding winds. Gravity also acts upon 
the sand which flies from the crests, causing it to fall across the stream lines of 
the air, the larger or heavier particles falling more steeply. To the varying density 
of the sand-shower is due the varying angle of the windward slope of dunes. 
When there is no sand-shower the windward becomes as steep as the leeward slope. 
When the dune tract is all deep sand the lower part of the eddy gouges out the 
trough, and, when the sand-shower fails, the wind by drifting and the eddy by 
gouging form isolated hills upon a hard bed. On the other hand, the sand-shower 
sometimes smooths over a dune tract, leaving lines of hollows (‘ Fuljes’), where 
the troughs were deepest and the wind strongest. The encroachment of a dune 
tract being due not only to the march of the dunes (by drifting) but also to the 
formation of new dunes to leeward from material supplied by the sand-shower, 
it follows that there is both a ‘group velocity’ and a ‘ wave velocity’ of dunes. 
Since the wave velocity decreases as the amplitude increases, a sufficiently large 
dune is a stationary hill, even though composed of loose sand throughout. Bind- 
ing the surface will stop the wave-motion, but not the group motion. Both may 
be arrested by promoting the growth of the dune. 
The fundamental forms of sand-dunes include the longitudinal, formed where 
_ the strength of the wind is too great to permit free lateral growth. Where the 
_ wind begins to decrease a form is met with intermediate between the longitudinal 
and the transverse. Conical dunes may be produced by the action of varying 
winds upon the rudimentary longitudinal dunes, called Barchanes. 
Where material is accumulated by the action of tidal currents, forms homolo- 
gous with the ground plan of dunes are produced. 
SATURDAY, SEPTEMBER 19. 
The following Papers and Report were read :— 
1. World Maps of Mean Monthly Rainfall. 
By Anprew J. Hersertson, /.R.SL., PRG. 
For practical purposes it is almost as important to know how rainfall is 
distributed throughout the year, as to know the total annual precipitation. The 
best way to show this is to make maps of mean monthly rainfall that will be 
comparable. Each month must be considered one-twelfth of a year and the 
1896, 3K 
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