The aqueduct of Lisbon has been long admired for the excellence of its construction, and, in point of magnitude, is not inferior to any similar edifice which the ancients have

left us. That part of it situated in the valley of Alcantara, about a mile from Lisbon, consists of thirty-five arches, by which the water is conveyed over a deep vale, formed by

two opposite mountains. The dimensions of it in the deepest part of the valley are as follows height of the arch from the ground to the intrados, 230 feet, 10 inches from : ;

the vortex of the arch to the extrados, exclusive of the parapet, 9 feet, 8 inches from the ;

extrados to the top of the ventilator, 23 feet, 4 inches making a total height from the ;

ground of 263 feet. Span of the principal arch, 107 feet, 8 inches breadth of piers of ;

the principal arches, 28 feet ; and thickness of the piers in general, 23 feet, 8 inches. The arches on each one diminish in breadth, as the piers side of the principal whereon they rest decrease in height, with the declivity of the ground they do not, how- ;

ever, decrease by any regular progression, neither are the curves employed in the arches of one kind, fourteen of them being in the Gothic or pointed style the rest are semi-cir- ;

cular, a variation adopted by the architect, Manuel dal Maga, from an apprehension, that

making the five principal arches semi-circular, he would considerably add to the expense. In the rest of the edifice, much judgment has been displayed no part of it has failed, or ;