Of the four great battles of the Persian Wars, Marathon is the only one in which a visit to the scene suggests no new problem. It is simple to reconstruct Thermopylæ on paper, but a survey of the spot disconcerts all one's calculations, owing to the changes wrought by the advance of the land upon the sea. The view of the straits of Salamis insists upon questions of orientation which hardly arise in studying the descriptions of Æschylus and Herodotus.