538 THE VAL DI CHIANA. 
points the flow of the streams which pour down into it, and 
there confining their waters by temporary dams until the sedi- 
ment was deposited where it was needed. The economical 
result of these operations has been, that in 1835 an area of 
more than four hundred and fifty square miles of pond, marsh, 
and damp, sickly low grounds had been converted into fer- 
tile, healthy, and well-drained soil, and, consequently, that so 
much territory has been added to the agricultural domain of 
Tuscany. 
But in our present view of the subject, the geographical 
revolution which has been accomplished is still more interest- 
ing. The climatic influence of the elevation and draining of 
the soil must have been considerable, though I do not know 
that an increase or a diminution of the mean temperature or 
precipitation in the valley has been established by meteorologi- 
cal 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 Arno a new tributary equal to the largest of its former 
afiiuents, 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 proportion, and, 
instead of augmenting the danger from its inundations, has 
almost wholly obviated that source of apprehension.* Between 
* 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 form- 
