CALCULATING THE STRENGTH OF TUBES. 201 



tube be only one-sixtcentli 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 conse- 

 quently 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, and would require to be doubled for 

 one hundred feet. A round tube, one incli in diameter, 

 having less surface to its sides, would be about one-third 

 stronger. A tube twice the diameter would need twice 

 tlie thickness ; or if less in diameter, a proportionate de- 

 crease in thickness 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 

 materials already referred to. 



SPRINGS AND ARTESIAN WELLS 



result from the upward pressure of water. Rocks are 

 usually arranged in inclined layers (fig. 227), and when 



Fig. 227. 



d 

 h ^ 



rain falls upon the surface, as at c c?. It sinks down in the 

 more porous parts between these layers, to c. If the lay- 

 ers happen to be broken in any place below, the water 

 finds its way up through the crevices by the pressure of 

 the head above, and forms springs. If there are no open- 

 ings through the rocks, deep borings are sometimes made 

 9* 



