of thrust of the arches, so that if a portion of the bridge were removed, the remainder of the arches and piers would maintain their By the present plan the permaposition.

nency of any one individual arch may be considered to depend upon that of the whole structure.*

The Aqueduct of Spoleto, has been standing about eleven hundred years and is still in a perfect state of preservation.

With proper care in preparing the foundations of the bridge at Harlem River, there is no good reason to fear that it will be less durable than that of Spoleto.

Aqueducts op Ancient Rome.

The largest and most magnificent Aqueducts of which we have any account, were the work of the Romans and ;

the ruins of several of them, both in Italy, and other countries of Europe, remain to the present time monuments of the power and industry of that enterprising people.

* It is proper to remark that, the pier at each extremity, of the range of arches

of eighty feet span, has an extra thickness, making it a pier of equilibrium ; this is

also the case with the one in the centre of that range of arches, so that on each shore

and in the centre of the river this additional security has been given.

For 440 years from the foundation of Rome the inhabitants contented themselves with the waters of the Tiber, and of the wells and fountains in the city and its neighborhood. But at that period the number of houses and inhabitants had so augmented, that they were obliged to bring water from distant sources by means of Aqueducts. Appius commenced this scheme of improvement. About 39 years after him, M. Curius Dentatus, who was censor with Papirius Cursor, brought water from the neighborhood of the city of Tibur ; and applied towards defraying the expense, part of the sums taken in the spoils of Pyrrhus.