Miscellaneous, 



66 



[July, 1911. 



sections appear unduly large for the 

 catchment, then it would appear that a 

 large proportion of the rainfall runs off 

 by these channels and comparatively 

 little is absorbed in the ground. There 

 are some localities where these pheno- 

 mena are very marked. If the beds of 

 the water-courses arb sandy, which they 

 usually are in moderately fiat country, 

 and where no rock is found on . or near 

 the surface, then it may be well to sink 

 pits in the sand (which costs little) and 

 see if the underground water is still 

 flowing. The result may be some indi- 

 cation of the porosity or otherwise of the 

 subsoil formation. In any case a deeply 

 cut river bed is in itself an elongated 

 trial pit, which shows the formation 

 down to this depth. If a fair supply of 

 water is found, after a dry spell, in the 

 sand bed, then it is fairly safe to sink a 

 well alongside (as shown in Diagram 

 No. 4); and if the sides of the river 

 banks show no seepage and the bed is 

 dry, this is a pretty safe indication that 

 well sinking at moderate depths is use- 

 less in that locality. In South Africa 

 farmers often provide a sort of artificial 

 water supply to wells by throwing up 

 low dams on lines transverse to the 

 direction of the natural surface slope of 

 the ground, the object being to form 

 small tanks or reservoirs in which some 

 of the surplus water, which would 

 otherwise have run off the surface into 

 the natural drainages, is held up. The 

 wells are sunk on the lower side of these 

 low dams, and the water plane is raised 

 above the surface, on the upper side of 

 the dams, and stands there in small 

 lakes. This tends to raise the water 

 plane on the lower side also by seepage 

 and prolongs the supply in the wells. 



A similar method is adopted in some 

 of the very dry districts in India also, 

 and the water which is thus stored for 

 irrigation is not drawn off direct by 

 sluices from the tanks, but is lifted from 

 wells sunk below the bunds. The rea- 

 son is that it is well known that where 

 the water is so raised by manual labour 

 it is not wasted, and a minimum is used. 



Tfce village and other tanks of Ceylon 

 have the same effect in raising the 

 water plane, and the landowners in very 

 dry districts take care to plant their 

 coconuts only under the bunds of tanks 

 or channels. Where wells are sunk, their 

 sites are invariably chosen in similar 

 positions. 



II. Any reference to wells generally 

 leads up to the question of artesian 

 systems, and perhaps a few remarks 

 may not be out of place in this connec- 

 tion, as the popular notions regarding 

 these are frequently tinged with the 

 magical and supernatural. 



The writer may be permitted to give 

 a very homely illustration of an artesian 

 system as follows. Imagine a large 

 basin or chatty (the kind called "thatti- 

 ya " in Sinhalese) partly filled with sand 

 and then another smaller basin or chatty 

 of exactly the same shape placed inside 

 the bigger one and pressed down into 

 the sand until the bottom reaches 

 within, say, one inch of the upper sur- 

 face of the bigger chatty, leaving about 

 one inch in thickness of a layer of sand 

 between the two chatties and all over 

 the outer surface of the smaller chatty. 

 Assume also that the size of the smaller 

 chatty is such that when it rests on this 

 one-inch thick bed of sand all round its 

 top edge will be just level with that of 

 the bigger chatty. Let the upper chatty 

 then be filled with some watertight 

 substance, such as putty, but the upper 

 surface of this putty be hollowed out by 

 a depression, which is about half the 

 depth of the chatty (if completely filled). 

 To make the whole appear more natural 

 the larger chatty may be buried in the 

 ground until its brim is just level with 

 the surface. We have here a miniature 

 artesian system, and the top edge of the 

 sand is usually called the " outcrop." 



If now a small tube equal in length to 

 the height from the centre of the hollow 

 formed in the putty to the sand at the 

 bottom be inserted in the centre of the 

 hollow formed in the upper surface of 

 the putty, and this tube be pushed down 

 until it reaches the smaller chatty and 

 is passed through a hole drilled in the 

 latter and into the sand at its lower end, 

 then the water held up in the sand will 

 rise in the tube, and under favourable 

 conditions will begin to flow out at the 

 top end and partly fill the hollow in 

 the putty. 



This is what happens in artesian 

 systems, and the reason is because the 

 " outcrop" cf the water-bearing stratum 

 of sand is higher than the bottom of the 

 hollow formed in the surface of the 

 putty. If the water supply to the sand 

 "outcrop" is kept up to meet the loss 

 in water drained off by the pipe, the 

 prosess will continue indefinitely. Dia- 

 gram No. 5 gives a rough idea of the 

 system.) In actual nature the under- 

 and over-lying watertight strata are 

 formed of clay or some such material, 

 and the water-bearing stratum lying 

 between them may be sand or gravel or 

 a mixture of both. The circle of the 

 outcrop may be tens or hundreds of 

 miles across, and one side may be 

 depressed and lie deep below the 

 surface, or the whole system may be 

 depressed. In the case where the whole 

 system lies deep below the surface, the 

 water will not naturally rise to the 



