THE VAL DI CIIIAlSrA. 519 



equare miles of pond, marsh, and damp, sickly low grounds Lad 

 been converted into fertile, healthy and well-drained soil, and> 

 consequently, that so much territory has been added to the agri- 

 cultural domain of Tuscany. 



But in our present view of the subject, the geographical revo- 

 lution which has been accompHshed is still more interesting. 

 The climatic influence of the elevation and draining of the soil 

 must have been considerable, though I do not know that an in- 

 crease or a diminution of the mean temperature or precij)itation 

 in the valley has been established by meteorological observation. 

 There is, however, in the improvement of the sanitary condition 

 of the Val di Chiana, which was formerly extremely unhealthy, 

 satisfactory proof of a beneficial climatic change. The fevers, 

 which not only decimated the population of the low grounds but 

 infested the adjacent hills, have ceased their ravages, and are now 

 not more frequent than in other parts of Tuscany. The strictly 

 topographical effect of the operations in question, besides the 

 conversion of marsh into dry surface, has been the inversion of 

 the inclination of the valley for a distance of thirty-five miles, 

 so that this great plain which, within a comparatively short 

 period, sloped and drained its waters to the south, now inclines 

 and sends its drainage to the north. The reversal of the currents 

 of the valley has added to the Amo a new tributary equal to the 

 largest of its former affluents, and a most important circumstance 

 connected with this latter fact is, that the increase of the volume 

 of its waters has accelerated their velocity in a still greater pro- 

 portion, and, instead of augmenting the danger from its inunda- 

 tions, has almost wholly obviated that source of apprehension.* 



* Arrian observes that at the junction of the Hydaspes and the Acesines, 

 both of which are described as wide streams, "one very narrow river is 

 formed of two confluents, and its current is very swift." — Arrian, Alex. 

 Anah., 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 cir- 

 cumstances, and especially upon the angle of incidence. If the two nearly 

 coincide in direction, so as to include a small angle, the joint current wUl 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- 



