Composition of Waters of Land- Drainage and of Rain. 125 



Cornwall, 38; Gloucester, 30; Lancashire, 34; Bute, 38i ; 

 Orkney, 41, These numbers would give us for the West of 

 Enijland, taken collectively, an average of something more than 

 364 inches per annum as the rain-fall. 



On the other hand the quantity falling in the eastern and mid- 

 land counties is thus given : — Suffolk 23];, Middlesex 25, Notting- 

 ham 25, Fife 31, Perth 24 — giving an average for these counties 

 of 25^ inches. But besides these general distinctions which apply 

 more or less to the whole of the two opposite sides of this island, 

 there are local variations of every imaginable character due to 

 height above the sea, the neighbourhood of hills, or to the form 

 which currents of air assume at different places, &c. &c. It is 

 notorious to every one that the quantity of rain which falls, and 

 the way in which it is distributed — whether in large quantity at 

 distant intervals, or in continually recurring showers — are dif- 

 ferent in almost every different locality. The period of the year 

 has also an influence on the quantity of rain that falls ; it is 

 generally greatest in the autumn and least in the spring. But 

 although, no doubt, »11 these circumstances would have to be 

 taken into account, if we were attempting a very accurate estimate 

 of the result of drainage, they need hardly trouble us here, since 

 some general and wide deductions are all that we are at liberty 

 to form from the data at our disposal. I shall assume for the 

 sake of argument a rain-fall of 25 inches, both because it is a 

 convenient number and because it fairly represents the quantity 

 observed in the districts from which my principal samples of 

 drainage-water were collected. 



By calculation we find that 25 inches of rain over an acre of 

 land is equal to 5<J7,1(>S gallons, or about 2532 tons a year — a 

 quantity which is enormous, but Avhich must be increased by 

 nearly one-half for the western counties and to a still greater 

 extent for Ireland. 



Let us now see what proportion of this amount finds its way 

 into the drains under ordinary circumstances. There have been, 

 no doubt, direct observations of the quantity of water running in 

 tlie drains of a given area of land, and 1 have found some of 

 those mentioned in different books. If you could do it, there is 

 no question that tlie surest wav of getting at this result would be 

 to gauge the drainage-water escaping from a certain number of 

 acres of land at the same time that you ascertained by the 

 ordinarv rain-gauge tlie Cjuantitv of rain falling. But such a 

 method" seems open to mucii doubt from tlie uncertainty as to 

 whether in some cases part of the water mav not esiape by other 

 means than the drains, or, on the other hand, its quantity be in- 

 creased by soakage from external sources. It appears to me that 

 the best nu)de of forming an (stimate on this subject, is that 



