Another important point with regard to this route is, that it was not merely the direct road to Delphi from the north, but the only direct road. As such it must have been in frequent use, and, in the flourishing days of the Delphi-Thermopylæ Amphictyony, of considerable importance. It was, of course, by reason of its very nature, liable to interruption by even a slight rise of the Asopos; and the frequent storms which traverse the Malian plain even in summer render such a possibility perennial. But even so it is a strange thing that in Greek military history it is never used as a means of avoiding direct assault on Thermopylæ.[112] [Illustration: THERMOPYLAE, FROM BRIDGE OF ALAMANA. _From Sketch by E. Lear._] 1. Site of Upper Drakospilia. 2. Mount Kallidromos. 3. Cliffs of West Gate. [_To face page_ 263. ] #THERMOPYLÆ AND ATREMISIUM.# The true significance of the step which the council at the Isthmus took in deciding to send a land force to Thermopylæ and the fleet to Artemisium can only be appreciated if the intimate strategical connection between the two places be realised. The dual nature of the Persian advance necessitated a dual form of defence. Had the invading force been merely a land army, the defence of the pass of Thermopylæ alone would have been sufficient; but it is evident that the Greeks fully appreciated the uselessness of attempting the defence of that pass, if the Persian fleet were left free to land a large body of troops in rear of it, and so turn the defence. Successful defence of Thermopylæ was then absolutely dependent upon successful defence of the Euripus. [Illustration: VIEW FROM THERMOPYLÆ, LOOKING TOWARDS ARTEMISIUM. 1. Malian Gulf. 2. Mount Othrys (circ. 15 1⁄2 miles). 3. Northern Branch of Euripus. 4. Site of Alpenoi. 5. Hills of North-west Eubœa. ] [Illustration: CHANNEL OF ARTEMISIUM, FROM 1600 FEET ABOVE THERMOPYLÆ. 1. Malian Gulf. 2. Mount Othrys. 3. Northern Branch of Euripus. 4. Hills of North-west Eubœa. 5. Euripus. [_To face page_ 264. ] #DEFENCE OF THE STRAIT A NECESSITY.# But there were other reasons which pointed not merely to the necessity of defending the Euripus, but of establishing the defence of the channel at the point at which it was established—at Artemisium. [Illustration: ON THERMOPYLÆ-ELATÆA ROAD (_viâ_ MODERN BOUDENITZA). _From Sketch by E. Lear._] 1. Mount Othrys. 2. Malian Gulf. 3. Deposit at Mouth of Spercheios. [_To face page_ 265. ] ◆Paus. x. 21.◆