Quantity of Water Consumed in Irrigation. 35 



charge of the Northern division ot the canal, has recorded 

 the results of some measurements of the volume of water 

 passing down it in December, 1860, from which the author 

 has calculated that the average daily loss in the first 15 

 miles was 2*66 feet in depth, and in 31 miles a little 

 further down it was probably 1J feet (see A pp. C). 



19. The following are some results calculated by the 

 author from data given by Colonel Bai'rd Smith, R.E., for 

 three canals in Italy : — * 





Total 



discharge 



per 



second. 



Loss 



in whole 



length 



per 

 second. 



Length 



of 

 canal. 



Mean 



wetted 



perimeter, 



estimated 



by the 



author. 



Average loss. 





Per mile 

 of canal 



per 

 second. 



Depth over 



wetted 

 surface in 

 24 hours. 



Naviglio Grande . . 

 Canal Muzza 

 ,, Martesana . . 



c ft. 



1851 



2652 



843 



eft. 

 158 

 477 

 105 



miles. 

 31 

 35 



28 



feet. 



100 



150 



60 



c. ft. 



5 



13$ 



3§ 



feet. 



•835 

 1-487 

 1-023 



These results are merely approximations, as the data are 

 incomplete; but even allowing a considerable margin for 

 errors, the results support the views of the author regarding 

 the excessive loss in certain soils. Some experiments made 

 by the author some years ago, to ascertain what proportion of 

 rainfall might be expected to flow off the ground, showed that 

 about a quarter-inch per hour (6 inches a day) was absorbed. 

 The soil was a light sandy one, in which, by the way, a very 

 fair garden was formed, and the subsoil to a depth of 

 3 or 4 feet was very similar in appearance to the surface soil. 

 More water would doubtless have been absorbed had a con- 

 stant head been maintained ; all that was done in the 

 experiment was to prevent the ground drying up. 



20. In the disposal of sewage there is a system called 

 " intermittent downward filtration/' in which sewage ■ is 

 poured in large quantities on land, with a view to its being 

 purified in its passage through the soil. The land must be 

 thoroughly underdrained. At Kendal, England, on one occa- 

 sion, sewage was flowing on to the land "at the rate of 

 2,000,000 gallons per diem, equivalent to a depth of 19 inches. 

 The average quantity of sewage flowing out of 

 the land was at least 1,000,000 gallons per diem, equivalent 



*Italian Irrigation, Second Edition, Vol. 

 270-276. 



I., pages 219-225, 250-254, and 



