1820.] Royal Academy of Sciences. 63 



the Pontine Marshes. Theodoric confided the draining of it to 

 Decius. Leo X. and Sextus V. caused several works worthy of 

 notice to be executed, but these were by no means to be com- 

 pared with the works executed from 1777 to 1796 under the 

 pontificate of Pius VI, who expended 9,000,000 of francs upon 

 them. Unfortunately, the plan had been laid on theoretical 

 views, veiy specious, and very seducing, proper indeed in many 

 respects, but being too general, the consequences were unfortu- 

 nate ; so that these works considered in a hydrauhc point of 

 view exhibit mere sketches of vast conceptions, in which many 

 parts of great importance are wanting, having been thought 

 imworthy of notice. Very circumstantial, historical, and critical 

 details upon all these objects are to be found in this work. 



By means of borers, it has been ascertained that the sea 

 formerly washed the feet of the mountains which form the eastern 

 and southern boundaries of the Pontine Marshes. The whole of 

 the phenomena concur in showing that the formation of these 

 marshes was caused, on the one hand, by brooks and torrents 

 running into a gulph which formerly covered the isles of Circe, 

 Zanoni, and Ponza ; and, on the other hand, by the sea forming 

 two ridges of sand banks, the last of which has at length shut 

 up the communication between the sea and the internal gulph. 

 Colmates's mtethod of employing currents of water charged with 

 mud to raise up the soil by its deposition and successive increase, 

 offers here only a secondary resource, as its effect is so very 

 slow. Notwithstanding its insufficiency, it will be proper to 

 continue the trials of this kind, which have been already begun. 

 And M, de Prony advises the use of it for the amehoration of 

 some soils, to which he beheves it to be very applicable ; but it 

 cannot be considered as any other than a subsidiary means. 

 The principal method to be used for the draining of these 

 marshes can only be a good system of canals to carry off the 

 water. To establish a system of this kind, it would be neces- 

 sary, in the first instance, to procure an exact plan of the 

 ground, its declivities, its torrents, its rivulets, the quantity of 

 rain that falls annually on it, and the quantity it throws oft" by 

 evaporation. This preliminary knowledge, being at present very 

 imperfect, M. de Prony has begun to fill up the parts that were 

 still wanting to complete it. By means of three signals placed 

 at known distances on the same straight line, he determined, in 

 the most expeditious manner, all the points from whence he 

 could observe his three signals. He has also reduced this 

 curious problem to general and convenient formulae. In fact, 

 this is only a particular case of the problem by which Hipparchus 

 determined the eccentricity and distances of the sun and moon. 

 Snell was the first that transferred this problem from the heavens 

 to the earth, and made use of it in surveying. Hipparchus's 

 problem has been reduced by us into general formului, which 

 comprehend the case used by M. Prony, We have been curious 



