12 



TILE DEAINAGE. 



quantity of salt brine upon the spot where the excretions had 

 been thrown. The water in the spring soon became percepti- 

 bly salt in taste. Fig. 3 illustrates this. 



Fig. 3. Imperfect filtration by coarse sand and gravel as water-bear- 

 ing strata. The spring that furnished water to the hamlet was at B; 

 the typhoid-soil pollution at A, about half a mile distant. Results as 

 given in the text. 



In this way gravel veins in clayey subsoils and artesian 

 wells are explained. The " head " of water is higher, and at 

 a distance, and is connected with the place where the arte- 

 sian well is dug, by a deep coarse sand or gravel stratum. 

 Fig. 4 illustrates this. 



A 



Fig. 4. Artesian well. A to D is a coarse gravel and sand stratum 

 between nearly impervious strata of clay. Artesian well B D, drilled 

 and tubed down to D. If it is tubed up to C, and the stratum A D is 

 saturated clear up to A, then the hydrostatic pressure will force the 

 water in the tube up to a level at C. If tubed only to the surface at B 

 it will make a flowing, or artesian well, 



