This is the genesis of the story; and, in carrying out my programme, I have endeavoured to convey by means of natural incidents and conversations between the characters portrayed, the most recent and reliable scientific information respecting the moon and Mars; together with other astronomical information: stating it in an interesting form, and in concise, clear, and understandable language. Every endeavour has been made to ensure that this scientific information shall be thoroughly accurate, so that in this respect the book may be referred to with as much confidence as any ordinary textbook. Apart from my own studies and work, all these facts have been carefully verified by reference, as regards the moon, to the works of such well-known authorities as Neison, Elger, Proctor, Sir Robert Ball, &c., whilst, with respect to Mars, the works of Professor Lowell, Flammarion, Professor Langley, and other writers, as well as practical papers by other actual observers of the planet, have been studied. The personal opinions expressed are entirely my own, and the technical writers above mentioned are in no way responsible for them. I do not, however, expect my readers to accept all my views, as they relate to matters in which there is ample room for differences of opinion. The reader will, of course, understand that whilst the astronomical information is, in all cases, scientific fact according to our present knowledge, the story itself--as well as the attempt to describe the physical and social conditions on Mars--is purely imaginative. It is not, however, merely random imagining. In a narrative such as this some matters--as, for instance, the "air-ship," and the possibility of a voyage through space--must be taken for granted; but the other ideas are mainly logical deductions from known facts and scientific data, or legitimate inferences. Many years' careful study of the various theories which have been evolved has convinced me that the weight of evidence is in favour of Professor Lowell's conceptions, as being not only the most reasonable but the most scientific; and that they fit the observed facts with a completeness attaching to no other theory. These conceptions I have endeavoured to present fully and clearly; together with my own views as an entirely independent writer.