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] * * * * * Briefly, having determined to take advantage of the approaching opposition of Mars in MDCCCXX ...

[3], I had my vessel constructed with walls three feet thick, of which the outer six and the inner three inches were formed of the metalloid.