One was the intense cold they were sure to experience. Where there was no terrene matter or moisture or air to retain the solar or astral heat that travelled through space, the diffusion of the streams of thermal energy would render any far voyaging from the earth impracticable. The experiments to meet this difficulty took three directions. One was physiological,--to make the body capable of resisting as great a degree of cold as they would be likely to encounter; this attempt was only partially successful, and that by slow steps. They brought themselves to live with pleasure in any cold that could be found in or around the earth; but it would take many centuries, perhaps geological ages, to bring endurance up to the pitch of interstellar cold; it would in fact mean such a sublimation of their bodies as would make them like spirits. Another direction was chemical,--to produce a regular atmosphere round the body as it flew, so that it might retain some of the streams of heat that swept past it; the use of the essence of the oxygen-plant helped them in this direction to some extent; but the amount of it that would be needed to keep up such an atmosphere for years, concentrate it as much as they liked, meant so huge a cargo that none of their winged cars would be able to bear it above the earth. The third direction was physical,--to produce as much heat around the body as would act as a shield against the cold of the ether; this was the most successful; for there were such torrents of energy ever moving through interstellar space that it merely needed its utilisation to solve the problem. One plan, that, when carefully developed, would ensure success, was a magnetic garment which would cover the whole of the body and draw to it all the electric energy within a large radius of it, to be transformed into heat by minute engines distributed all over the envelope. Another was, to combine the mechanical collection of electricity from the ether and the full development of the magnetic powers of the body. Already they had been able to flash lightnings around them as they flew through the night; and it would need but small mechanical manipulation to increase this display and to turn it into heat. Like meteors, they would blaze across space, wrapped in a mantle of flame.