DESIGNING. 163 



available natural elevation of 22 or 23 ft., representing a pres- 

 sure of say 10 Ibs., the difference between this and the re- 

 quired pressure of 61.2 Ibs. is 51.2, and which we see (page 

 65) represents a head of 120 ft. approximately; and we there- 

 fore determine to erect a stand-pipe 120 ft. in height, and, 

 having assumed the height, the capacity required fixes the 

 dimensions. 



The question of capacity is settled most arbitrarily; but, 

 in general, it is the usual practice to provide a storage or 

 reserve supply which will permit the temporary stoppage of 

 the pumping-engines for repairs, etc., for a given number of 

 hours. In small towns, particularly where a lighting-plant 

 may be operated in conjunction with the water-works, it is 

 sometimes deemed desirable to provide sufficient storage to 

 supply the ordinary consumption during the day by the pump- 

 ing done at night, making only one set of firemen and 

 engineers necessary for both plants. Another determining ele- 

 ment in fixing the capacity of storage and the corresponding 

 size of the reservoir is, of course, the item of cost and the 

 amount of money available. As has been shown, the widest 

 range of practice in the matter of diameter, height, and cor- 

 responding capacity exists; but, for the purpose of discussion 

 and analysis, we will assume that a metallic reservoir of 

 400,000 U. S. gals, is required. The height having been 

 taken as 120 ft., from the table (page 95), we see that, for the 

 given height and capacity, the diameter will be approximately 

 24 ft., the actual capacity for the cylinder, 120 X 24 ft., being 

 407,150 U. S. gallons. 



Strain-sheet. In designing such a structure, through the 

 employment of the principles previously enunciated, the 

 details can be specified ; their correctness demonstrated mathe- 

 matically, or shown graphically. 



Usually a graphic demonstration of the correct principles 

 of construction is shown by a." strain-sheet," similar to that 



