Thus the oceans of the world, as yet mere watery deserts, useful to hardly a calculable percentage of the people (and then only at the expense of the rest) will have become the world's inheritance, and its hoarded wealth will stave off the time--whose coming we must not ignore--when our world-capital begins to be exhausted. For that time must come. We are living upon the hoards which the womb of our mother the earth has borne to our father the sun. But our mother is, in respect at all events of mineral wealth, past the age of conception; and every century brings us more rapidly near to the time when we shall, like spendthrifts, have lived out our capital. Already the end of coal is in sight. When, at the end of a vista however long, we begin to be able to foresee the exhaustion of other minerals, we shall face a problem appalling in its nature. Perhaps before our store of heat gives out and reduces earth to the state of a dead world like the moon, we shall already have exhausted our stock. No economies in the use of scrap metal and the re-employment of the material of machines which have been superseded can save us from ultimate metallic bankruptcy in a future calculated perhaps in thousands (but not many thousands) of years. Our only succour seems to lie in a conception for which (despite the efforts of some lively thinkers who have been obliged to ignore all but the least important difficulties of the subject) we have no material--the conception of means by which the cold depths of interplanetary space may be traversed. Even if we allow imagination, untrammelled by the most evident necessities of the case, to suggest a speed of transport computable only by astronomical analogies, we still lag behind anything which could serve this purpose, unless we concurrently believe that human life shall, by that time, be lengthened into centuries. Otherwise, however recklessly we may conceive of speed in interplanetary travel, man would almost require to live for many centuries in order to reach and return from any destination which would not inevitably destroy him by fire or cold when he arrived at it. Most likely man is for ever destined to accept the bounds of his own planet, and to be limited by its resources. In order that these resources may be utilised to the uttermost of his needs, the contents of the ocean floor must undoubtedly be laid under contribution, and probably we shall not antedate this achievement if we consider that it will have been at least entered upon a hundred years hence. CHAPTER VII THE MARCH OF SCIENCE