The above words, ’every day,’ require an explanation. Of course, once they were really out in ’the realms of starry space,’ there were really no alternations of day and night, for the sun shone upon them continuously. But within the aerostat artificial nights, so to speak, were produced by drawing huge screens across the semi-transparent outer casing. Mr Monck explained this to the young voyagers, giving them, incidentally, a little lecture, as it were, in astronomy and general science; and on this occasion he had as his auditors all four of the young passengers—including, that is to say, the two cousins, Silas and Freddy. ’I expect you all know,’ he said, ’that out in what is called space, where there is no atmosphere—no air—the sun’s rays seem to have no heat. The cold there is most intense—far greater than anything ever experienced upon Earth. You feel the sun’s rays warm on your globe because they pass through the Earth’s atmosphere, which acts like a lens or magnifying-glass. Here the same effect is obtained by passing them through the wonderful semi-transparent metal of which the outer shell of the airship is composed. It is harder than the hardest steel, yet almost transparent like glass, without being brittle, while it is far lighter than aluminium. It was discovered by King Ivanta, and is called "ivantium" after him. He found that when the sun’s rays were passed through it the result was exactly the same as when they pass through the atmosphere of the Earth or of Mars. That is how it is we are so warm and comfortable on board here. But for the discovery of that metal such a journey as we are taking would be impossible. We should be frozen to death.’ ’Then there is no need to have day and night unless you like,’ Freddy observed, his blue eyes opening in surprise. He was a fair, good-looking youngster, and a great favourite with Monck and the chums. ’No, my lad. But King Ivanta considers it best to keep up the same habits as those you and his people are all accustomed to "at home;" for Mars turns on its axis in about the same time as the Earth—namely, twenty-four hours or thereabouts. That is to say, the average day on Mars is just about the same length as the average day on the Earth.’ On many other occasions, when he had the time and opportunity, the good-natured engineer ’trotted them round’ and explained to the young people, in similar fashion, the why and the wherefore of many of the things that puzzled them—so far, that is, as he himself understood them. But as to a great many, and those some of the most surprising, he was obliged to confess his own entire ignorance.