380 Utilisation of Waste Substances, ^t., on the Farm. 
rural district is very small. So small is it, in point of fact, that 
if we can conceive of the whole volume required in the rural 
districts to be taken from it, the full volume of the " wants of 
the district," before it was absorbed by the earth, it would 
scarcely be appreciated, and assuredly would not in any way 
interfere with the requirements of " the general water economy " 
of the district or country. Mr. Bailey Denton, in fact, estimates — 
and his estimates are always characterised by judicious careful- 
ness — the amount required to be only one-seventyfourth of the 
rainfall. I have doubled the amount required, or estimated to 
be required, per head per day of the population, which is surely 
liberal enough. But when 1 say that the usual estimate is Jai' 
in excess of that which experience shows to be actually used, 
one will perceive what small ground there is for holding the 
opinion that the rainfall of any district will fall short of all 
the demands made upon it, either by domestic requirements, 
or the general water economy of the country. But it may be 
said that these calculations, however convincing, refer to the 
general rainfall on the earth's surface, but give no data for 
the case of roofs. Refraining again from giving figures, and 
giving in place a fact, from a pretty wide range of experience 
it appears to be a rule that in the case of a domestic structure, 
the proportion the roof-surface bears to the rainfall is amply 
sufficient to give a supply of water for the consumption of 
the family which the roof covers. And although the range of 
experience in the case of farm-buildings has been of necessity 
much narrower, still it would appear as if the same rule happily 
existed. The coincidence is more than curious, and ought not 
to be forgotten in thinking of utilising rainfall on roof-surfaces. 
It is pleasing to know that the utilising of these is becoming 
daily more and more frequent ; some of the plans adopted 
being characterised by an ingenuity of adaptation in existing 
circumstances not often met with in other departments of 
economy. 
From what has now been given by way of opening up the 
general subject, the reader will be able to perceive how 
numerous are the points which it involves, how interesting their 
details, how practical may be their outcome, and yet how, not- 
withstanding all this, much of which is widely known, there 
has been a profound indifference to it, the reason for which it 
has been difficult to account for. Recent experiences have, 
however, given some clue to this, and it is at least a most 
suggestive circumstance that practical and technical science 
has, in taking up and in bringing such wonderful results out 
of the subject of the utilisation of waste substances, again 
shown the way by which the best can be made of all the 
