North of that river the coast is low-lying and bordered by sand-lunes, to which succeed on the Strait of Dover the cliffs in the neighborhood of the port of Boulogne and the marshes and sand-dunes of Flanders, with the ports of Calais and Dunkirk, the latter the principal French port on the NOrth Sea.
Right angled at C, semicircles are described on the three sides, thus forming two lunes Afcda and Cgbec. The sum of the areas of these lunes equals the area of the triangle ABC.] As for Euclid, it is sufficient to recall the facts that the original author of prop. 8 of book iv.