146 ON THE ARTIFICIAL MODES 



Caroline. But the level of the sea varies according to 

 the tides, being many feet more elevated at high water 

 than at ebb tide ? 



Mrs. B. The medium must therefore be taken as a 

 general level ; and, raising the most elevated canals above 

 that, let off the water at ebb tide into the sea : the canals 

 are furnished with locks, which are then closed, to prevent 

 the water returning when the tide flows. The mode used 

 in Holland to raise the water from the lower to the upper 

 canals consists of a species of small windmills: as the 

 wind blows regularly, though not violently, in that country, 

 they perform their office very well. The same process of 

 windmills is adopted in Cambridgeshire arid Bedfordshire. 



Emily. Pray can you explain to us the mode in which 

 the valley of Chiana in Tuscany, which was anciently an 

 unwholesome marsh, has been brought to such a beautiful 

 state of cultivation as it now exhibits ? 



Mrs. B. The marshes in Tuscany are formed by the 

 waters which flow from the Apennines, and which, not 

 finding a sufficient vent, are arrested in their course, and 

 becomes stagnant. The Apennines being of a loose sandy 

 texture, the waters bring with them a great quantity of 

 earth, which they deposit in the lowest parts to which 

 they flow. 



Caroline. I recollect that the Arno, when swollen by 

 rain, is quite thick with mud, brought down from the 

 mountains by the torrents which feed it. 



Mrs. B. The celebrated Torrecelli took advantage of 

 the deposition of this mud to invent a mode of draining 

 the marsh of the Chiana. But before I explain the rem- 

 edy, it will be necessary to inform you whence the evil 

 arose, and give you an account of the origin of this marsh. 



In ancient times, the numerous rivulets which flow 

 from the adjacent hills into the valley of Chiana poured 

 their united waters from thence into the Tiber. Some 

 inconvenience being experienced by the Romans, from 

 occasional overflowing, they constructed a dyke to close 

 this outlet. The waters, thus stopped in their course, 

 formed a lake, which, when raised by accumulation to a 



810. How is the level of the sea situated! 811. What is the 

 mode of raising water in Holland'? 812. How are the marshes of 

 Tuscany formed'? 813. Who invented the mode of draining the 

 marsh of the Chiana 1 814. In what manner was the lake first form- 

 ed? 



