ARTESIAN WELL 



223 



the bottom of the water-bearing beds. In this case the quantity of water which can 

 be obtained by the borehole is necessarily limited. A large quantity of water may, 

 however, be obtained R _ 



under these condi- 

 tions (which are illus- 

 trated by fig. 87) 

 should the water - 

 bearing strata be very 

 porous, and have a 

 considerable lateral 

 extension. 



2nd. On the other 

 hand the inclination 

 of the strata may be 

 very gradual, as shewn 

 in Jig. 88, and in this 

 case the area of sur- 

 face receiving the 



rainfall is much greater. This is due not only to the surface of the water-bearing 

 rocks having a larger superficial area, but also to the fact that near to the surface 

 the overlying rocks are generally found to be more open than they are at a consider- 

 able depth from the surface. Hence, whilst in fig. 87 the rainfall from z to Y is the 



8S 



most that can be expected to reach the borehole, in fig. 88 it is probable that most of 

 the rainfall on the area from c to D will percolate through to the water-bearing strata. 

 This is the condition under which the largest quantity of water may be expected to 

 bo obtained in the prosecution of Artesian wells. 



3rd. Another condition is where a boring has to be made to water-bearing strata 

 through other rocks, which, though compact in their nature, are not impermeable. 

 This condition naturally affects the quantity of water obtainable, and in such a case 

 it is important to obtain increased hydrostatic pressure. 



4th. In some cases several qualities of water may be met with in one boring, some 

 of which may be found chemically objectionable. When satisfactory water has been 

 found, the impure water can only be kept back by the insertion of tubes in the bore- 

 hole. With the third condition mentioned above, the application of tubes will also 

 sometimes be found advantageous. They are always necessary where running sand 

 or very loose strata are met with. 



5th. Another case occurs when the water met with by a boring has so little 

 hydrostatic pressure that it will not rise in the borehole to the surface. In this case 

 the water has to bo raised, when its level is within 30 feet of the surface, by some 

 description of pump. When the level of the water is at a greater distance than this 

 from the top, a plunger-pump has to be used. 



In cases 1 and 2 a bed of impermeable rock is assumed to intervene between the 

 surface and the water-bearing strata. It need hardly be mentioned that the quantity 

 of Tvater found in any class of strata does not depend only on the surface of such 

 strata exposed to the rainfall, but is much influenced by the degree of porosity of the 

 strata, which is the test of its saturativo capability. 



An illustration of this is afforded by the results of the sinkings of many of the 

 deepest coal-mines in Great Britain, where such sinkings have passed through the 

 Permian beds before reaching the coal-measures. Between the magnesian limestone 

 and coal-measures is found in nearly every instance a bed of red sand, varying in 

 thickness from a few inches to 12 feet. Whilst feeders of water of a few hundred 

 gallons per minute only have been encountered whilst passing through the limestone, 

 the feeders met with on reaching the more porous sand-bed referred to, have fre- 

 quently been enormous, in several instances amounting to over 4,000 gallons per 

 minute. In such cases the quantity of water can doubtless be traced to the principles 

 indicated on fig. 88. 



Under the conditions referred to in the fifth head, may be mentioned cases where 

 water may be obtained by short holes, bored a few yards into the ground, the object 

 being to collect the surface-drainage, and the water being obtained by email pumps. 



