of the temple of Syene (Assuan), in Egypt. At this place there was a religious festival on the day when the sun shone vertically at noon, and consequently threw no shadows. One year Eratosthenes happened to be in Alexandria on the day in question, and as this town lies 2000 stadia to the north of Syene, the sun's rays were not vertical, but threw shadows. The angle the vertical made with the sun's rays gave the angular distance between Alexandria and Syene, which Eratosthenes found to be 7⅕ degrees. Assuming the earth to be a perfect sphere, 2000 stadia divided by 7⅕ gives the length of 1 degree of circumference, and this, multiplied by 360, gives the total circumference of the earth. This calculation is sufficiently accurate for most purposes; actually the earth is not a perfect sphere but is flattened at the poles; roughly, however, we may say that the distance from the surface to the centre is 4000 ml.; of this 30 ml. is the crust, with which we, as geologists, have to deal.
Besides being flattened towards the poles the earth's shape is irregular, in that the Pacific Ocean forms a great flattened depression on that side of the globe, as in the thick end of a pear, while the continent of Africa forms a mass projecting above the normal surface on the op-
Fig. 1. Eratosthenes' Measurement of the GlobeThe sun's rays being parallel, when the sun is vertical at Assuan, A, the walls of houses will throw shadows at Alexandria, B. The angle the sun's rays make with the walls of the houses is equal to the angular distance ACB. Knowing the distance AB, this divided by the number of degrees of the angle ACB and multiplied by 360 gives the length of the circumference of the globe.
