208 THE EMPTY QUARTER

problem presented by them is of the same order as that of
the desert booming. Yet Mr. C. Carus-Wilson, who is an
expert in such matters, failed to get any response whatever
from my specimen1 of the ' Singing Sands' of Naifa by
methods which set samples from Forth Oer and the Isle of
Eigg squeaking like a host of crickets. Whether the Forth
Oer sands would ever boom if piled up on steep Naifa-like
cliffs and set in motion I am not competent to say. The Naifa
sands certainly do not boom or squeak underfoot or when
kicked, and it may be that the Welsh sands would not squeak
or boom if pushed down a steep slope. Or it may be indeed
that, while only particular sands squeak at a touch, all
sands would boom under suitable conditions.2

Such questions are only for the experts. But there are
points about the Rube al Khali sands which may be suggested
for consideration. The Forth Oer sands squeak even when
their subsurface is moist, provided that the actual surface
is dry, but the Naifa sands had no music in them at 7 a.m.
nor again, as I was to discover later on, after a comparatively
light sprinkling of rain. Yet they boomed mildly at 7 p.m.
after producing their full tone the same afternoon. Climatic
conditions generally—humidity, temperature, wind, etc.—
appear therefore to have some bearing on the subject.

In the next place the Naifa sands behaved differently in
the various zones of the musical sand cliff. The moving mass
began by grating at the top and continued by booming in the
middle, while they ceased to emit any sound at all on the
cessation of movement on the slope. Movement would
therefore seem to be an essential cause both of the grating
and the singing. It is difficult to say whether the volume
of the sliding mass is greater at the start or when the singing
begins. It is true that the sands, when they slide, set the
sands before them in motion, but the friction certainly stops
part of the original mass on the way while the final stoppage
of movement actually takes place on a steep incline. The
presumption is therefore that, while the mass gathers volume

JIt should perhaps be admitted that the quantity of sand available for
his experiments was too small for the purpose,
* See note by Dr. Vaughan Cornish in Appendix.