368 THE EMPTY QUARTER

velocity of 25 miles (1-32 x 1C5 feet) per second possesses, accord-
ing to the equation e = Jww;2, a kinetic energy measured by
l-95x!015 foot-pounds. If this mass is suddenly stopped the
energy of motion will be transformed into heat and, according to
Joule's mechanical equivalent of heat, would yield 2-51 x 10la
British thermal units—that is, sufficient to raise the temperature
of 2*51 x 1012 pounds of water 1° F. But we are dealing with
limited quantities of iron and quartz sand, the specific heats of
which at higher temperatures have been determined as 0*22 and
0*25 respectively. Dividing this amount of heat between the
100 tons of iron and an equal mass of sand, the temperature
developed would be of the order of 2-38 x 107° F. or 13,000,000° C.
Even with a velocity of only 10 miles per second the temperature
would be about 200,000° 0.

This result seems absurd, and no doubt many factors have been
overlooked in this simple calculation. But what one puts into
the mathematical machine, that one gets out. The heat developed
would be more than sufficient to vaporize the whole mass ; but
the shock was instantaneous and there would not be time for the
conduction of heat to the centre of the mass of iron, before it,
together with the surrounding ground, was shot out by the gases
generated. Any moisture in the sand and underlying rocks
would be instantly converted into steam, but probably the gases
developed by the vaporization of iron and silica would be still
more effective in producing a tremendous * back-fire.'

The sharply developed Widmanstatten structure shown by the
larger mass of the Wabar iron proves that this portion of the mass
was not raised to a temperature above 850° C. The partial de-
struction of this structure and the development of a granular
structure seen in the smaller piece does, however, indicate that
such a temperature was here reached. It is clear that the pieces
of iron that we have for examination are merely remnants of a
much larger mass or masses. That some iron and silica were
actually vaporized is proved by the structures shown by the
' bombs * of silica-glass.

Fortunately such meteoritic falls with the formation of craters
are of rare occurrence on the earth's surface, though it has been
suggested that they have been more frequent on the moon.
With the possible exception of the Siberian fall on June 30th,
1908, about which little is yet known, none has occurred within
historic times. London would be completely wiped out by such a
fall. In fact few examples of meteorite craters are yet known as
topographical features of the earth's surface. Mr. Philby's
observations and the examination of the material he collected at
Wabar have thrown much light on the problem. The largest