520 EESULTS OF OPERATIONS. 



Between the beginning of the fifteenth century and the year 

 1761, thirty-one destructive floods of the Arno are recorded; 

 between 1761, when the principal streams of the Yal di Chiana 

 were diverted into that river, and 1835, not one.* 



Hesults of Ojperations. 



It is now a hundred years since the commencement of the 

 improvements in the Yal di Chiana. Those of the Maremma 

 have been in more or less continued operation for above a gen- 

 eration. They have, as we have seen, produced important geo- 

 graphical changes in the surface of the earth and in the flow of 

 considerable rivers, and their effects have been not less conspicu- 

 ous in preventing other changes, of a more or less deleterious 

 character, which would infallibly have taken place if they had 

 not been arrested by the improvements in question. 



posite, directions, the velocity of the principal branch will be retarded both 

 above and below the junction, and at high water it may even set back the cur- 

 rent of the affluent. 



On the other hand, the diversion of a considerable branch from a river re- 

 tards its velocity below the point of separation, and here a deposit of earth in 

 its channel immediately begins, which has a tendency to turn the whole stream 

 into the new bed. " Theory and the authority of all hydrographical writers 

 combine to show that the channels of rivers undergo an elevation of bed below 

 a canal of diversion." — Letter of Fosse mbroni in Salvagnoli, Baccolta di 

 Documenti, p. 33, See the early authorities and discussions on the principle 

 stated in the text, in Frisi, Del modo di regolare i Fiumi e i Torrenti, libro iii., 

 capit. i., and Mengotti, Idraulica, ii., pp. 88 et seq., and see p. 486, rwte, ante. 



In my account of these improvements I have chiefly followed Fossombroni, 

 under whose direction they were principally executed. Many of Fossom- 

 broni's statements and opinions have been controverted, and in comparatively 

 imimportant particulars they have been shown to be erroneous. — See Lom- 

 BAEDrNi, Chiida alio studio delV Idrohgia, cap. xviii., and same author, Esame 

 degli Studi ml Tevere, § 33. 



* Fossombroni, Memorie Idraulico-storiclie, Introduzione, p. xvi. Between 

 the years 1700 and 1799 the chroniclers record seventeen floods of the Arno, 

 and twenty between 1800 and 1870, but none of these latter were of a properly 

 destructive character except those of 1844, 1864 and 1870, and the ravages of 

 that of 1870 were chiefly confined to Pisa, and were occasioned by the burst- 

 ing of a dike or wall. They are all three generally ascribed to extraordinary, 

 if not unprecedented, rains and snows ; but many inquirers attribute them to 

 the felling of the woods in the valleys of the upper tributaries of the Arno 

 Bince 1885. See a paper by Grifpini, in the Italia, Nuova, 18 Marzo, 1871. 



