Page:Popular Science Monthly Volume 16.djvu/509

Rh from other sources. With Eratosthenes, who died about the beginning of the second century before Christ, the science of geography may be said to have begun. He was the first to apply a purely scientific method to ascertain the magnitude of the earth; for, when a knowledge of the exact circumference of the globe was once obtained, the different countries and places could be arranged in these ancient maps, in their relative position to each other, far more accurately. The distances between places in what was then known as the inhabited part of the earth were previously ascertained by the number of days it took to go from one place to another, derived from the information of travelers and mariners.

To rectify the errors which became more apparent and confusing as the inhabited part of the world became better known, Eratosthenes devised, what has ever since been employed as the most accurate means of determining the circumference of the earth, the measurement of an arc of the meridian. He found a confirmation of the globular form of the earth in the fact that at Syene, in Upper Egypt, upon the tropic, the sun at noon on the day of the summer solstice was vertical—that is, that it cast no shadow, a well at the bottom being enlightened by its rays; while at Alexandria, upon the same day and time, it was distant from the zenith one fiftieth of the circumference of the circle. Eratosthenes obtained by this means the length of what is called an arc of the meridian, or a portion of the curved surface of the earth; and from this he was able, by a familiar rule, to determine the circumference of the whole circle.

The happy idea occurred to Hipparchus of applying to the earth the same method he had used in fixing the position of the stars in the celestial sphere. Regarding the earth as a great circle, which, like any other circle, is divisible into three hundred and sixty degrees, he so divided it, by lines of circles drawn perpendicularly from the poles to the equator, and by parallel lines at equal distances from the equator to the poles, which was the beginning of the division of the globe by lines of longitude and latitude into degrees.

The Romans, in their representation of the earth, at first followed Eratosthenes and Hipparchus. The Emperor Augustus ordered the geographers and designers to prepare for the use of the people a map of the habitable world which should represent fully the extent of the Roman Empire; and, from some fragments that were preserved, it is known that this map was a cylindrical projection of a great circle. The Romans, however, had a map for practical use, which they styled a descriptive itinerary, or, as they sometimes called it, "painted roads." This map was in the form of a band, about a foot wide and about twenty feet long, upon which the habitable earth was continuously represented along parallel spaces. It represented pictorially the great routes or roads of the empire, the position of places with the distances between them, the ranges of mountains, and the direction of