Represent to the mind on one and the same plane-surface Iberia and India with the intermediate countries, and define likewise the west, the east, and the south, which are common to every country. To a man already acquainted with the arrangement and motions of the heavens, and aware that in reality the surface of the earth is spherical, although here for the sake of illustration represented as a plane, this will give a sufficiently exact idea of the geographical [position of the various countries], but not to one who is unacquainted with those matters. The tourist travelling over vast plains like those of Babylon, or journeying by sea, may fancy that the whole country stretched before, behind, and on either side of him is a plane-surface; he may be unacquainted with the counter-indications of the celestial phenomena, and with the motions and appearance of the sun and stars, in respect to us. But such facts as these should ever be present to the mind of those who compose Geographies. The traveller, whether by sea or land, is directed by certain common appearances, which answer equally for the direction both of the unlearned and of the man of the world. Ignorant of astronomy, and unacquainted with the varied aspect of the heavens, he beholds the sun rise and set, and attain the meridian, but without considering how this takes place. Such knowledge could not aid the object he has in view, any more than to know whether the country he chances to be in may be under the same latitude as his own or not. Even should he bestow a slight attention to the subject, on all mathematical points he will adopt the opinions of the place; and every country has certain mistaken views of these matters. But it is not for any particular nation, nor for the man of the world who cares nothing for abstract mathematics, still less is it for the reaper or ditcher, that the geographer labours; but it is for him who is convinced that the earth is such as mathematicians declare it to be, and who admits every other fact resulting from this hypothesis. He requests that those who approach him shall have already settled this in their minds as a fact, that they may be able to lend their whole attention to other points. He will advance nothing which is not a consequence of these primary facts; therefore those who hear him, if they have a knowledge of mathematics, will readily be able to turn his instructions to account; for those who are destitute of this information he does not pretend to expound Geography. 2. Those who write on the science of Geography should trust entirely for the arrangement of the subject they are engaged on to the geometers, who have measured the whole earth; they in their turn to astronomers; and these again to natural philosophers. Now natural philosophy is one of the perfect sciences.[704]