Of a Country on the Climate. 



219 



Practical Applications: Reservoirs. — In surveying the 

 drainage area of a river, with a view to ascertain the actual 

 available supply, it is absolutely necessary to have the most 

 accurate information respecting the geological formations; 

 not only to determine the amount of surface-water which 

 may reach the river basin, but to learn what portion of the 

 rainfall is retained in the strata, which can also ultimately 

 flow thither. In this country, if the hills are very high, they 

 will retain a certain proportion of water, which may either 

 flow to the river basin, or descend to great depths. 



We have immense floods in winter, and almost dry beds in 

 summer ; and hence it is exceedingly difficult to compute the 

 mean annual discharge of any river. It would, manifestly, 

 be wrong to compute the mean annual discharge, by taking 

 the lowest summer level and the highest winter level : the 

 latter might represent the condition of the river for one day 

 only, and the former for 200 days out of the 365. 



The only discharge on which a calculation can be founded 

 for practical purposes, is the lowest, or rather the average 

 summer level. It is upon this supply that we must depend, 

 most certainly, if the requirement is for sanitary purposes ; 

 for it would be manifestly highly injurious to the health of a 

 town to store surface-water, in a climate such as this, in an 

 open reservoir. Let me express myself more clearly on a 

 point which is aU-important in Austraha. The summer cur- 

 rent of the river must be carefully measured, and it ought to 

 bear a certain proportion to the consumption. For example, 

 if the consumption is 100,000 gallons per diem, it would be 

 well to have a permanent stream, equal at least to 25,000 

 gallons ; but in this proportion I would rather indicate a 

 principle than assert a fact. If the surface-water is collected 

 where basaltic rocks abound, the supply of running water 

 should be proportionably greater, because the soil, in such 

 localities, is charged with decomposing vegetable matter. If, 

 on the contrary, sandstone rocks abound, a smaller supply of 

 pure water might suffice to render the drainage water 

 innoxious. 



When I attach so much importance to the summer stream, 

 I do not mean to assert that it is impossible to collect the 

 winter rains, or that such a method is always objectionable. 

 It is only objectionable when it is largely in excess of the 

 permanent current. Setting aside all difficulties attending 

 the formation of an impervious reservoir, it is exceedingly 

 hazardous to rely upon winter rains. The climate of Victoria 

 is very peculiar. For many months in the year we have 



