EFFECTS OF AQUEDUCTS AND CANALS. 447 
a profuse abundance of water.* On the other hand, the filtra- 
tion of water through the banks or walls of an aqueduct car- 
ried upon a high level across low ground, often injures the 
adjacent soil, and is prejudicial to the health of the neighbor- 
ing population; and it has been observed in Switzerland and 
elsewhere, that fevers have been produced by the stagnation of 
the water in excavations from which earth had been taken to 
form embankments for railways. 
If we consider only the influence of physical improvements 
on civilized life, we shall perhaps ascribe to navigable canals a 
higher importance, or at least a more diversified influence, than 
to aqueducts or to any other works of man designed to control 
the waters of the earth, and to affect their distribution. They 
bind distant regions together by social ties, through the agency 
of the commerce they promote; they facilitate the transporta- 
tion of military stores and engines, and of other heavy material 
connected with the discharge of the functions of government ; 
they encourage industry by giving marketable value to raw ma- 
terial and to objects of artificial elaboration which would other- 
wise be worthless on account of the cost of conveyance; they 
supply from their surplus waters means of irrigation and of 
mechanical power; and, in many other ways, they contribute 
much to advance the prosperity and civilization of nations. Nor 
are they wholly without geographical importance. They some- 
times drain lands by conveying off water which would other- 
wise stagnate on the surface, and, on the other hand, like aque- 
ducts, they render the neighboring soil cold and moist by the 
percolation of water through their embankments; + they dam 
* The unhealthiness of the Roman Campagna is ascribed by many medieval 
as well as later writers to the escape of water from the ancient aqueducts, 
which had fallen out of repair from neglect, or been broken down by enemies 
in the sieges of Rome. 
+ Sismondi, speaking of the Tuscan canals, observes: ‘‘ But inundations 
are not the only damage caused by the waters to the plains of Tuscany. 
Raised, as the canals are, above the soil, the water percolates through 
heir banks, penetrates every obstruction, and, in spite of all the efforts 
of industry, sterilizes and turns to morasses fields which nature and the 
