teaching of history is that rivers connect rather than separate. Strategical reasons have almost invariably been responsible for their conversion into Frontiers. As States developed and considerable armies were required for their defence, the military value of rivers, in delaying an enemy, and in concentrating defensive action at certain bridges, or fords, or posts, became apparent, and in the demarcation of larger kingdoms and States, they provided a convenient line of division, everywhere recognizable, and easily capable of defence. It was for this reason that Augustus selected rivers—the Rhine and the Danube—as the Frontiers of the Roman Empire, though strategical considerations soon tempted the Romans beyond, as the English have been tempted across the Indus, and the French by other causes across the Mekong.
Accordingly the advantages and disadvantages of rivers as Frontiers may be thus stated. The position of the river is unmistakable, no survey is required to identify or describe it, and the crossing-places frequently admit of fortification. Rivers are lines of division as a rule very familiar to both parties, and are easily transferred to a treaty or traced on a map. On the other hand, they may be attended by serious drawbacks, confronting diplomatists and jurists with intricate problems. Rivers are liable to shift their courses, particularly in tropical countries. The vagaries of the Helmund in Seistan, where it is the boundary between Persia and Afghanistan, have led to two Boundary Commissions in thirty years. The precise channel which contains the Frontier line, the division of islands, very likely new accretions, in the river-bed, the determination of drinking rights or of water-rights in cases where cultivation is only
