SPRINGS AND ARTESIAN WELLS. 179 



DETERMINING THE STRENGTH OF PIPES. 



The question may now arise, and it is a very import- 

 ant one, How thick must be a lead tube of this size to 

 prevent danger of bursting with a head of fifty feet, or 

 of any other height ? To answer it, let us turn to the 

 table of the Strength of Materials in a former part of 

 this work, where we find that a bar of cast lead one 

 fourth of an inch square will bear a weight of fifty-five 

 pounds. If the tube be only one sixteenth of an inch 

 thick, one inch of one of its sides will possess an equal 

 strength, that is, will bear fifty-five pounds only, and 

 the tube would consequently burst with fifty feet head. 

 If one tenth of an inch thick, the tube would just bear 

 the pressure, and, to be safe, should be about twice as 

 thick, or one fifth of an inch. Half this thickness 

 would be sufficient for twenty-five feet of water, which 

 would require to be doubled for one hundred feet. A 

 round tube, one inch in diameter, having less surface 

 to its sides, would be about one third stronger. A tube 

 twice the diameter would need twice the thickness ; 

 or if less in diameter, a proportionate decrease in thick- 

 ness might take place. If, instead of cast lead, milled 

 lead were used, the tube would be nearly four times 

 as strong, according to the table of the strength of ma- 

 terials already referred to. 



SPRINGS AND ARTESIAN WELLS 



Result from the upward pressure of water. Rocks are 

 usually arranged in inclined layers {Fig. 150, p. 180), 

 and when rain falls upon the surface, as at c d, it sinks 

 down in the more porous parts between these layers, 



