attractive planet Mars, I determined to begin with that voyage of
which the conditions and the probable result were most obvious and
certain. I preferred, moreover, in the first instance, to employ the
apergy as a propelling rather than as a resisting force. Now, after
passing beyond the immediate sphere of the Earth's attraction, it is
plain that in going towards Mars I should be departing from the Sun,
relying upon the apergy to overcome his attraction; whereas in seeking
to attain Venus I should be approaching the Sun, relying for my main
motive power upon that tremendous attraction, and employing the apergy
only to moderate the rate of movement and control its direction. The
latter appeared to me the more delicate, difficult, and perhaps
dangerous task of the two; and I resolved to defer it until after I
had acquired some practical experience and dexterity in the control of
my machinery.

It was expedient, of course, to make my vessel as light as possible,
and, at the same time, as large as considerations of weight would
admit. But it was of paramount importance to have walls of great
thickness, in order to prevent the penetration of the outer cold of
space, or rather the outward passage into that intense cold of the
heat generated within the vessel itself, as well as to resist the
tremendous outward pressure of the air inside. Partly for these
reasons, and partly because its electric character makes it especially
capable of being rendered at will pervious or impervious to the
apergic current, I resolved to make the outer and inner walls of an
alloy of ..., while the space between should be filled up with a mass
of concrete or cement, in its nature less penetrable to heat than any
other substance which Nature has furnished or the wit of man
constructed from her materials. The materials of this cement and their
proportions were as follows. [2]