reckon
on evolving 5,000 cubic feet of gas in an hour, using his elements
in the following proportions: water, 560 lbs.; sulphuric acid (sp. g.
1.85), 144 lbs.; iron turnings, 125 lbs. The gas, as given off, was
cooled and purified by being passed through a head of water kept cool
and containing lime in solution. Contrasted with this, we find it
estimated, according to the practice of this time, that a ton of good
bituminous coal should yield 10,000 cubic feet of carburetted hydrogen
fit for lighting purposes, and a further quantity which, though useless
as an illuminant, is still of excellent quality for the aeronaut.

It would even seem from a statement of Mr. Monck Mason that the value of
coke in his day largely compensated for the cost of producing coal gas,
so that in a large number of Green's ascents no charge whatever was made
for gas by the companies that supplied him.

Some, at least, of the methods formerly recommended for the management
of free balloons must in these days be modified. Green, as we have seen,
was in favour of a trail rope of inordinate length, which he recommended
both as an aid to steering and for a saving of ballast. In special
circumstances, and more particularly over the sea, this may be reckoned
a serviceable adjunct, but over land its use, in this country at
least, would be open to serious objection. The writer has seen the
consternation, not to say havoc, that a trail rope may occasion when
crossing a town, or even private grounds, and the actual damage done
to a garden of hops, or to telegraph or telephone wires, may be very
serious indeed. Moreover, the statement made by some early practitioners
that a trail rope will not catch so as to hold fast in a wood or the
like, is not to be relied on, for an instance could be mentioned coming
under the writer's knowledge where such a rope was the source of so much
trouble in a high wind that it had to be cut away.

The trouble arose in this way. The rope dragged harmlessly enough along
the open ground. It would, likewise, negotiate exceedingly well a single
tree or a whole plantation, catching and releasing itself with only such
moderate tugs at the car as were not disturbing; but, presently, its
end, which had been caught and again released by one tree, swung free
in air through a considerable gap to another tree, where, striking a
horizontal bough, it coiled itself several times around, and thus held
the balloon fast, which now, with the