Volume II
may be a torrent whose normal flow is only a small amount of water, but when the floods come, a very large amount is brought down, and so it gets much broader, and the level of the spring gets lower. Now round arches can not be made because they would be very thick and occupy too much room, and they would cost too much as well. But water has to pass regularly, and the distance is great. So I think no better invention could be found for this purpose, for nothing else could rise but little and yet any way have a wide space for floods, unless it be these square arches. For they leave a space free and unobstructed so that even if it rises the water will find nothing to knock against, [/fol. 83r] since its path will not get narrower as it would with round arches at every little flood, so it would knock against the arches, and offer much greater resistance. M is the side from which the water should come, and' N O the lower part. (Illustration 52)
Crossing a spring or an aqueduct over round arches: this is to be done in a place which in season carries a great quantity of water, in time of rain, which then comes down with great impetus: to make this footbridge so it can resist the burden of so much accumulated water, an obtuse angle should be made in the centre, since that has greater strength than any other, because the water will strike against the pillars in the corner. So it is all made up of obtuse angles, the water never takes it on the flat, [/fol. 83v] and is cut up by those pillared corners, whereas if the aqueduct were to stand in a straight line the water would take the corners of the piers on the flat, with much greater force than it does if it is this way. (Illustration 53) The way to make aqueducts. They are made of different kinds, as can be seen by the inventions, since most are derived from Ancient works, although I have not made mention of particular places. In this way all the variations can be seen. What is to be noted most is that all have diminished arches as they go up. They are so built that each pillar [/fol. 84r] corresponds equally to a pillar of the second order, one lays its load on firm ground, and the next transfers its load to the air, since its load is borne in the centre of the arch. Almost all those that I have seen are of rustic work without any elaboration. The pilasters of the first arches ought to be not less than a third of the width between the pillars, nor thicker than a quarter. If the arches are doubled, let the upper and lower ones be of the same width. There is no need for them to be so high; let them be one quarter lower, or shorter, than the lower ones. In dividing the arches, care should be taken never to put a [186]