daytime, the temperature usually falls according to a general law; but
there are found regions where the fall of temperature becomes arrested,
such regions being commonly, though by no means invariably, associated
with visible cloud. It is probable, however, that it would be more
correct not to interpret the presence of cloud as causing manifestation
of cold, but rather to regard the meeting of warm and cold currents as
the cause of cloud.

The writer has experimented in the upper regions with a special form
of air thermometer of great sensibility, designed to respond rapidly
to slight variations of temperature. Testing this instrument on one
occasion in a room of equable warmth, and without draughts, he was
puzzled by seeing the index in a capillary tube suddenly mounting
rapidly, due to some cause which was not apparent, till it was noticed
that the parlour cat, attracted by the proceedings, had approached near
the apparatus. The behaviour of this instrument when slung in the clear
some distance over the side of the balloon car, and carefully watched,
suggests by its fitful, sudden, and rapid changes that warmer currents
are often making their way in such slender wandering rills as have been
already pictured as permeating the broader air streams. During night
hours conditions are reversed. The warmer air radiated off the earth
through the day has then ascended. It will be found at different
heights, lying in pools or strata, possibly resembling in form, could
they be seen, masses of visible cloud.

The writer has gathered from night voyages instructive and suggestive
facts with reference to the ascent of air streams, due to differences
of temperature, particularly over London and the suburbs, and it is
conceivable that in such ascending streams may lie a means of dealing
successfully with visitations of smoke and fog.

One lesson taught by balloon travel has been that fog or haze will come
or go in obedience to temperature variations at low levels. Thus thick
haze has lain over London, more particularly over the lower parts, at
sundown. Then through night hours, as the temperature of the lower air
has become equalised, the haze has completely disappeared, but only to
reassert itself at dawn.

A description of the very impressive experience of a night sail
over London has been reserved, but should not be altogether omitted.
Glaisher, writing of the spectacle as he observed it nearly forty years
ago, describes Londo