l899-'00 TRANSACTIONS 137 



of I, is of course not absorbed to any great extent, but flows 

 down to the gravelly surface between i and 2, and together with 

 the rain falling on this surface, is absorbed by the gravel, and 

 finds storage in it between the rock (R) and the clay (C). 

 Here it accumulates in a second and lower water-bearing 

 stratum, and the normal level for this water supply may be re- 

 presented by the line N. Now it is evident that if any of the 

 wells Wi to W* be continued through the clay, they will obtain a 

 water supply from this lower gravel ; and the well W'* can get a 

 permanent supply from no other source. W"* then becomes an 

 illustration of a so-called 'deep well,' and in wet seasons, when 

 the level rises above the normal N, say as high as Ni, this well 

 will overflow, or become what is called a ' flowing well,' on the 

 principle that water rises to the same height in all tubes con- 

 nected with a common reservoir. 



If I have succeeded in making my subject understood, we 

 are now prepared to begin the special enquiry that I wish to 

 propose for your consideration. Up to this point, I have merely 

 defined and illustrated certain terms that I shall have to use 

 repeatedly in the sequel. To recapitulate briefly, I have spoken 

 of three classes of wells, viz. : those fed from the normal ground- 

 water, those fed from a deep or secondary water supply, and 

 those so-called surface wells, which, like W^ in Fig. 3, receive 

 soakage water only, and contain a supply only when the ground 

 in the immediate vicinity is wet, as in Spring and Autumn. All 

 the ground-water wells diagrammed, are represented as dug 

 down to the subjacent claj^ ; but this is not a necessary condi- 

 tion. W^ for example, would be none the less a ground-water 

 well, had it been made no deeper than W^ and W^. We have 

 now to consider the character of the water which fills these three 

 types of wells. 



Rain water is not the chemically pure substance which it 

 is often described as being. In the later stages of a prolonged 

 rainfall, it is indeed very nearly pure, but the first portions of 

 every shower wash out from the air, not only the gaseous im- 

 purities , which are the products of animal and vegetable decay 

 — (Ammonia, compound or organic ammonias, sulphuretted 

 hydrogen, etc.) — but also those solid particles, rich in microbial 

 life, which form the dust of the air, and are partly organic and 

 partly inorganic in character. This rainwater, falling upon the 



