A WHIRLWIND AND WIND-RUSH AT GOSFIELD. 9
quantity of brown dust. This seems strange in view of the
fact that the ground traversed was mostly pasture and very
wet, owing to the thunder-storms earlier in the day. Some
of this dust Was gathered, perhaps, from the ground below
the trees, which were uprooted. This can hardly have been
the case, however, with the dust which the Messrs. Ardley
saw carried up actually at the place of origin of the storm and
immediately after it began. The extensive transport of dust
and dirt was observed also (as will be remembered) in the case
of the Writtle storm.
It is curious that two of these very-striking phenomena should
have occurred, no more than sixteen miles apart, within a period
of no more than twenty-one months; also that both should
have followed a more-or-less north-easterly course.
DISCUSSION.
Mr. F. J. W. Whipple read the following comments, written by Sir
Napier Shaw, F.R.S., Chairman of the Meteorological Committee:—
"So far as I can make out, this whirl arises from some local instability
in the upper air, which gives rise to a local rotation sufficiently rapid to
balance a distribution of pressure with a low pressure core. This dis-
tribution of pressure is then transmitted to the surface and the air is
gradually sucked out of the core and rotation set up. When the core
reaches the surface, it is very bad for the surface, because the core is
protected by spin, except just at the surface, and there the low pressure
at the centre has to be supplied by air which passes along the surface
and gets up spin as it goes. In so far as the core is fed at all, it is by what
moves along the surface, which cannot get up enough spin to make a
balance. So the arrangement becomes very like a vacuum cleaner on a
large scale, with a "hose" made of air spinning so fast that it can resist
the external pressure. Passing over water, the air going to the centre
blows the water into spray, which is carried to the low pressure in the
middle."
"The mechanical process at the bottom is thus sufficiently evident,
but the mechanism of the top is still unknown. There is something there
which keeps up a suction and so maintains a low pressure along the core
of the hose. The effect can be imitated by a fan-wheel in a solid lid or
by a gas-jet in a tube leading from a solid lid; but nobody has yet shown
how to set up a whirl of this kind when there is no lid. One can describe
possible conditions that would meet the case, but cannot realise them
experimentally, because one cannot work on a sufficiently-large vertical
scale."
Mr. Whipple also showed lantern-slides illustrating the damage done
to buildings by a tornado in America. He invited members who were
so fortunate as to have the opportunity to observe such phenomena to com-
municate at once with the Meteorological Office. Great interest attached
to the movements of the clouds during the passage of the tornado. It