Page:Vitruvius the Ten Books on Architecture.djvu/198

162 of plane trees, with walks laid out in them among the trees and resting places there, made of "opus signinum." Behind the "xys­tus" a stadium, so designed that great numbers of people may have plenty of room to look on at the contests between the athletes.

I have now described all that seemed necessary for the proper arrangement of things within the city walls.

1. subject of the usefulness of harbours is one which I must not omit, but must explain by what means ships are shel­tered in them from storms. If their situation has natural advan­tages, with projecting capes or promontories which curve or return inwards by their natural conformation, such harbours are ob­viously of the greatest service. Round them, of course, colon­nades or shipyards must be built, or passages from the colon­nades to the business quarters, and towers must be set up on both sides, from which chains can be drawn across by machinery.

2. But if we have a situation without natural advantages, and unfit to shelter ships from storms, it is obvious that we must pro­ceed as follows. If there is no river in the neighbourhood, but if there can be a roadstead on one side, then, let the advances be made from the other side by means of walls or embankments, and let the enclosing harbour be thus formed. Walls which are to be under water should be constructed as follows. Take the powder which comes from the country extending from Cumae to the promontory of Minerva, and mix it in the mortar trough in the proportion of two to one.

3. Then, in the place previously determined, a cofferdam, with its sides formed of oaken stakes with ties between them, is to be driven down into the water and firmly propped there; then, the lower surface inside, under the water, must be levelled off and