A continued passage runs through the centre of it, for the people who constantly attend to keep it in order, and a semi-circular channel or conduit of 13 inches diameter at each through which the water is conveyed. It is worthy of remark, that side, these channels are laid not in an inclined direction, as in other aqueducts, but horizontally to compensate for this, a small depression is made, at certain intervals, by which the ;

water is impelled along the horizontal line a manner, supposed by the natives, to require ,

less declension in conveying water than an inclined line. There are two thoroughfares for foot passengers along the aqueduct, one at each side of the corridor, which is 5 feet

70 PREL1MINARYESSAY.

wide, defended by a stone parapet. From some remains of the Romans on this spot, it is supposed they either did, or attempted to, construct an aqueduct in this situation. The honor of erecting this noble structure is due to John V. This munificent prince laid the foundation of it in 1713, and in thirteen years the whole was completed.

The city of Lisbon, in testimony of gratitude raised an arch to his memory, bearing a flattering inscription, in the Latin tongue, importing that he had not only conferred

wealth, glory, strength and peace on his kingdom, but that, overcoming nature as it were, he had introduced perennial waters into the city, and hailing him therefore as the best of princes, and the author of a great and useful public work.