number) which they bisect, as twenty feet span and fifteen feet high, separated by piers eleven feet thick. There is thus a triple promenade on this remarkable bridge--the vaulted passage below, the roadway and lateral galleries above, and the open footpath at the top of all. I should add that the upper part of the bridge is of brick, the piers and towers are of stone.”[118] There is no European structure akin to this, but for a long time Rothenburg on the Tauber has been famous for a two-storeyed bridge; also we know that some modern commercial bridges have an upper road and a lower one, like the High Level Bridge at Newcastle. In every case the idea was suggested by the Roman practice of building aqueducts in tiers. IV And now let us give all our attention to the more military bridges. Brangwyn has studied them with the utmost care and interest; there are but few variations of the war-bridge that his art has not yet represented. Let us see, then, what his research has found. 1. This bridge from Bhutan has the same technique as the cantilever bridges of Kurdistan (p. 74); but the gateway towers mark an advance. They are militant works, partly because they control the traffic, and partly because they are open below the eaves for archery and for other defensive warfare. Brangwyn suggests that gateway towers of this kind may have been brought to India by Darius Hystaspes (512 B.C.) or by Alexander the Great (327 B.C.). On this point there is no evidence. On the other hand, there seems to be no doubt that the timber gateways on Roman bridges in England, as in Gaul, were prepared for defence, though their main use was to limit the freedom of a public thoroughfare, invariably after sunset, and during the day in times of unrest. This was the first aim of defensive bridges, so the gateway towers in Bhutan are suggestive things to study. They are too light in structure to give us an idea of the bold and stern gateways built by the Romans with newly-felled trees; but yet they help us to realise vaguely what every young civilization must have done when it learnt from a free use of bridges that foes as well as friends were eager to pass without danger across rivers. [Illustration: PRIMITIVE TIMBER BRIDGE IN BHUTAN, INDIA]