"As soon as the balloon quits the earth, it is subjected to the
influence of many circumstances tending to create a difference in
its weight; augmenting or diminishing its ascending power. For
example, there may be a deposition of dew upon the silk, to the
extent, even, of several hundred pounds; ballast has then to be
thrown out, or the machine may descend. This ballast being
discarded, and a clear sunshine evaporating the dew, and at the
same time expanding the gas in the silk, the whole will again
rapidly ascend. To check this ascent, the only recourse is, (or
rather _was_, until Mr. Green's invention of the guide-rope,) the
permission of the escape of gas from the valve; but, in the loss
of gas, is a proportionate general loss of ascending power; so
that, in a comparatively brief period, the best-constructed
balloon must necessarily exhaust all its resources, and come to
the earth. This was the great obstacle to voyages of length.
