THE VAL DI CHIANA. 539 
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 Val di Chiana were diverted 
into that river, and 1835, not one.* 
ed of two confluents, and its current is very swift.”—Anrrian, Alex. Anad., 
vi., 4. 
A like example is observed in the Anapus near Syracuse, which, below the 
junction of its two branches, is narrower, though swifter than either of them, 
and such cases are by no means unfrequent. The immediate effect of the 
confluence of two rivers upon the current below depends upon local circum- 
stances, and especially upon the angle of incidence. If the two nearly coin- 
cide in direction, so as to include a small angle, the joint current will have a 
greater velocity than the slower confluent, perhaps even than either of them. 
If the two rivers run in transverse, still more if they flow in more or less op- 
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 afiluent. 
On the other hand, the diversion of a considerable branch from a river 
retards 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 FOSSOMBRONI, in SALVAGNOLI, 
Raccolta di Documenti, p. 32. See the early authorities and discussions on 
the principle stated in the text, in Frist, Del modo di regolare ¢ Fiumi e 7 
Torrenti, libro iii., capit. i., and Moneorrt, /draulica, ii., pp. 88 et seqq., and 
see p. 498, note, ante. 
In my account of these improvements I have chiefly followed Fossombroni, 
under whose direction they were principally executed. Many of Fossombroni’s 
statements and opinions have been controverted, and in comparatively unim- 
portant particulars they have been shown to be erroneous.—See LOMBARDINT, 
Guida allo studio dell’ Idrologia, cap. xviii., and same author, Hsame degli 
Studi sul Tevere, § 33. 
* FossoMBRONI, Memorie Idrautico-storiche, 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 were of a properly destruc- 
tive character except those of 1844, 1864, and 1870, and the ravages of this 
latter were chiefly confined to Pisa, and were occasioned by the bursting 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 since 
1855. See a paper by GRIFFINI, in the [talia Nuova, 18 Marzo, 1871. 
