As the whole heaven is separated into two parts by its equator, it follows that the earth must, by its equator, be similarly divided. The two hemispheres, both celestial and terrestrial, are distinguished into north and south. Likewise the torrid zone, which is divided into two halves by the equator, is distinguished as having a northern and southern side. Hence it is evident that of the two temperate zones, one should be called northern, the other southern, according to the hemisphere to which it belongs. The northern hemisphere is that containing the temperate zone, in which looking from east to west, you will have the pole on your right hand, and the equator on the left, or, in which, looking south, the west will be on the right hand, and the east on the left. The southern hemisphere is exactly the contrary to this. It is clear that we are in one or other of these hemispheres, namely, the north; we cannot be in both: “Broad rivers roll, and awful floods between, But chief the ocean.”[707] And next is the torrid zone. But neither is there any ocean in the midst of the earth wherein we dwell, dividing the whole thereof, nor yet have we any torrid region. Nor is there any portion of it to be found in which the _climata_ are opposite to those which have been described as characterizing the northern temperate zone. 4. Assuming these data, and availing himself likewise of astronomical observations, by which the position of every place is properly determined, whether with respect to the circles parallel to the equator, or to those which cut these latter at right angles, in the direction of the poles, the geometer measures the region in which he dwells, and [judges of the extent of] others by comparing the distance [between the corresponding celestial signs]. By this means he discovers the distance from the equator to the pole, which is a quarter of the largest circle of the earth; having obtained this, he has only to multiply by four, the result is the [measure of the] perimeter of the globe. In the same manner as he who takes the measures of the earth, borrows the foundation of his calculations from the astronomer, who himself is indebted to the natural philosopher, so in like manner the geographer adopts certain facts laid down as established by the geometer, before setting forth his description of the earth we inhabit; its size, form, nature, and the proportion it bears to the whole earth. These latter points are the peculiar business of the geographer. He will next enter on a particular description of every thing deserving notice, whether on land or sea; he will likewise point out whatever has been improperly stated by those who have preceded him, especially by those who are regarded as chief authorities in these matters.[708]