142 
MAGAZINE OF SCIENCE AND ART.' 
source in mountain ranges. In countries like tho 
North of Italy, where the rivers which discharge them¬ 
selves mto the Po have their sources in tho Alps timid 
perpetual snows, or like the North of India, where the 
streams descend from the still more lofty ranges of the 
Himalayas, thero is never any cessation in the supply, 
'—a warm summer in the mountains melts a larger 
quantity of snow arid produces a large supply, while 
a cold summer has tho effect of reducing it; but tho 
streams always discharge a laTge body of water, and all 
that is required is the construction of such works as 
may' enable the engineer to have at his command a part 
or even the whole of this discharge, so that auy required 
amount may be turned into the proper, channels by 
which tho water is to be conveyed to the districts whore 
it is to bo used. 
I have said that the most effective source of supply is 
from tho rivers which are fed from perpetual snows on 
mountain ranges ; hut I only mean that the sonreo is 
most effective because it is the largest in amount and 
most to he depended on. Any stream which will fur¬ 
nish a supply of water, however small, during the sum¬ 
mer months, can be made available for irrigation, and 
there are thousands of instances in England where the 
water of a moro brook is made to produce the most 
beneficial results upon the cultivation of thermal! val¬ 
leys through which it runs; but these sources occa¬ 
sionally fail in dry seasons, that is, at the very period 
when there is the greatest demand for water, whereas 
the river heading into tho mountains will at that very' 
time he able to furnish a full supply. 
The works which are required to make supplies from 
running streams available are, first—dams across these 
streams, by which the water is raised to such a height 
as will allow of any given amount being turned into 
the channel or carrier by which it is to he led into the 
district where it is to be applied. Where the rivers are 
large, and where tho water derived from it is intended, 
as fn the case of the Ganges Canal to supply the wants 
of a very large district, the stability of the dam on 
which tiro supply is dependent is of immense import 
ance; hut the larger the river the less generally will 
be the height of the dam, and the less subject will it 
therefore bo to the risk of accident from .floods, against 
which it is necessary always to take special precautions. 
Great care is required in selecting the site of a dam, 
the foundation must he solid, or capable of being made 
so_the banks should be high enough to retain tho 
heaviest floods, and should form solid abutments for 
(he ends of tho dam. 
If care be required in selecting the site of a dam, no 
less care should be exhibited in the selection and adap¬ 
tation, of the materials of which it is constructed.. 
These should be of the most solid and enduring de¬ 
scription, and should be framed and connected together 
in such a manner as to bid defiance to the action of the 
heaviest flood; and as water acts in various ways upon 
a structure of this kind, partly by direct pressure, partly 
bv erosion in passing over its surface, or by "bringing 
other bodies such as trees and stones to roll over it, and 
partly by acting upon the foundation upon which the 
dam "is bnilt—all these various actions must bo foreseen 
and guarded against. 
The second class of works which are required are 
those by which the water is conveyed and distributed. 
These are in fact mere water-courses of dimensions cor¬ 
respondent to the amount of water to be conveyed along 
them—care, of course, must be taken to regulate the 
slope of tile channel of these courses, so as not to create 
a current so rapid as to act on. the bed nr banks. 
Bridges are required to carry roads across these chan¬ 
cels ; aqueducts to pass the water across valleys; waste 
weirs to get rid of any surplus water ; culverts to pro¬ 
vide for the drainage of the country in the upper side 
of the carrier. In carrying out an extensive scheme of 
irrigation, there will be a demand for such an amount 
of skill and intelligence as will be found only among 
men who possess the highest order of engineering talent. 
Means, also, must be provided for measuring as accu¬ 
rately as possible tho quantity of water which is passed 
into the distributing channels, as on the appliances for 
this purpose, and the care taken to secure them from 
being tampered with, must the revenue to be derived 
fiom tbe water depend. Upon this revenue must be 
charged not only the cost of maintaining the works in 
proper repair, but also the interest of the capital ex¬ 
pended in their construction. 
The following abstract of some of the information 
collected by Captain Baird Smith,. of the Bengal En¬ 
gineers, during a professional tonr in the north of Italy, 
when ho was sent by the East India Government for 
the express purpose of collecting information on the 
subject of iirigation, will he both interesting and in¬ 
structive. in Lombardy the amount of land under 
irrigation exceeds 1,000,000 acres—about one-sixth of 
the whole area of the country ; the canals and branches 
by which the water is distributed are about 4.'.00 miles 
in length ; about half the w ater of the rivers which flow 
through the country is made use 'of for irrigation. A 
discharge at the rate of one cubic foot per second will 
irrigate 70 acres. The purchase of this amount of water 
in perpetuity costs from £290 to .£300; tho annual 
rent of the same amount of water is £13 10s., or about 
3s. lOd. or 4s. per acre. The actual benefit or direct 
return to the Government, which is the constructor and 
maintainor of the works, does not exceed tho amount 
required to keep the works in repair; but the indirect 
benefit, which is measured by the increased value given 
to the land, of which the annual rent is the index, may¬ 
be put at 12s. per acre, about £000,000 per annum. 
In Lombardy, where tho works have been carried on 
for centuries, and where every description of labour has 
been employed, it is of course impossible even to guess 
at tbo total expenditure upon the various kinds of 
works which have been executed for the purpose of irri¬ 
gation. Much has been done in tho earlier periods, 
upon bad principles and imperfectly ; much was com¬ 
menced and after a time neglected and. allowed to get 
out of repair, so -that it would be impossible to say 
whether thero has been a return in the snap® of interest 
upon the -whole of the capital expended ; hut in certain 
instances there is distinct information to be obtained 
with regard to particular works, which is sufficient to 
show the amount of benefit which is to be derived from 
a well-digested and carefully executed system of irri¬ 
gation. 
' Tho canal of C'aluso, in Piedmont, has a .course of 
nearly 20 miles—it supplies water for irrigating 17,955 
acres’; it is not well laid out, as tho slope of the bed is 
far too great, causing a waste of water and of mill- 
power. It is expensively constructed ; that is, there 
are many works of importance which have been neces¬ 
sarily undertaken—such as a heavy river wall at the 
commencement, 000 feet in length, two long tunnels, 
aqueducts, bridges, &c. The total cost of the work was 
£31,908 5s., or, on an average, about £1700 per mile. 
The amount of water available for irrigation is 350 
cnbic feel per second. The direct returns in the shape 
of rent for the use of water amount to £1780. The. 
annual expenses of repairs and maintenance do not 
amount to above £500, and which leave £1280 as the 
net revenue, being about 4 per cent, on the capital in¬ 
vested. The indirect returns, however, are estimated 
at 12s. per acre upon all the land irrigated, together 
with a share of tbe returns from milks, and this will 
amount to about £11.000 per annum, or about 30 per 
cent, per annum on the outlay. 
1 do not think it necessary to enter into further de¬ 
tails at present with relation to the mode in which the 
water of running streams is made available for irriga¬ 
tion. I may mention, however, that I have reason to 
believe that tbe revenue derived from irrigation in 
India is far in excess of that mentioned as the feturn 
from the works in Italy, and the indirect return is fully 
as great as in that country. 1 have, however, requested 
to be furnished by the proper authorities with full de¬ 
tails of the works' which have been executed, or are in 
progress of execution, in Bengal and Madras, and with 
