consequently in gas, the one being exactly answerable to the other,
and is therefore not calculated for voyages of long duration. Human
strength appears to be too feeble for great results, and moreover,
requires repose; which reduces the amount assignable to each man to a
fraction of its nominal value. Of electro-magnetism as yet we know
too little to enable us to pronounce upon it with certainty. Of the
remaining powers known only one is worth mentioning in connexion
with this subject, namely, the elastic force of air; and this I only
mention because it has been taken up by one whose authority in these
matters is deservedly entitled to much weight, and who entertains
great hopes of making it ultimately subservient to the purpose in
view.

But although none of these powers, in their present state, be so
perfectly adapted to the propulsion of the Balloon as to leave
nothing further to desire, yet are some of them so far applicable as,
undoubtedly, to enable us to accomplish, by their means, a very large
amount of success. A steam engine of the power required, namely, equal
to fourteen horses, could be easily constructed, far within the limits
of weight which we have at our disposal upon that account in the
Balloon under consideration, or even in one much smaller; and recent
improvements have so far reduced the amount of coal required for its
maintenance, that perhaps as long a voyage could be made by means of
it now, as would be expected or required. Even human strength, by
a certain mode of applying it, might be made effectual to the
accomplishment of a very sufficient rate of motion, say fourteen or
fifteen miles an hour, for, continuously, as long a period as the
natural strength of man, moderately taxed, could endure, and which we
may reckon at twelve hours.