to reflect it, simply spends itself in the air.
Then, always and under all conditions of atmosphere soever, there ensues
absolute silence until the time for the echo back from earth has fully
elapsed, when a deafening outburst of thunder rises from below, rolling
on often for more than half a minute. Two noteworthy facts, at least,
the writer has established from a very large number of trials: first,
that the theory of aerial echoes thrown back from empty space, which
physicists have held to exist constantly, and to be part of the cause of
thunder, will have to be abandoned; and, secondly, that from some cause
yet to be fully explained the echo back from the earth is always behind
its time.

But balloons have revealed further suggestive facts with regard to
sound, and more particularly with regard to the varying acoustic
properties of the air. It is a familiar experience how distant
sounds will come and go, rising and falling, often being wafted over
extraordinary distances, and again failing altogether, or sometimes
being lost at near range, but appearing in strength further away. A free
balloon, moving in the profound silence of the upper air, becomes an
admirable sound observatory. It may be clearly detected that in certain
conditions of atmosphere, at least, there are what may be conceived to
be aerial sound channels, through which sounds are momentarily conveyed
with abnormal intensity. This phenomenon does but serve to give an
intelligible presentment of the unseen conditions existing in the realm
of air.

It would be reasonable to suppose that were an eye so constituted as to
be able to see, say, cumulus masses of warmer air, strata mottled with
traces of other gases, and beds of invisible matter in suspension,
one might suppose that what we deem the clearest sky would then appear
flecked with forms as many and various as the clouds that adorn our
summer heavens.

But there is matter in suspension in the atmosphere which is very far
from invisible, and which in the case of large towns is very commonly
lying in thick strata overhead, stopping back the sunlight, and forming
the nucleus round which noisome fogs may form. Experimenting with
suitable apparatus, the writer has found on a still afternoon in May, at
2,000 feet above Kingston in Surrey, that the air was charged far more
heavily with dust than that of the London streets the next day; and,
again, at half a mile above the city in the month of August last