64 SMALL WATER SUPPLIES. 



tions 13, or 14 and 16 added together. The term 

 " Head " is the way in which water pressure is de- 

 noted in place of pounds per sq. in. or other units. 

 For instance, imagine a column of water i sq. ft. in 

 sectional area and 20 ft. high ; at the lowermost point 

 of this column the pressure due to the water would 

 be 20 ft. head, exerted equally in all directions, and 

 as water weighs 62^ Ibs. per foot cube the total pres- 

 sure on an area of i sq. foot at the bottom is equal 

 to 20 x 62*5 = 1250. As to the intensity of pressure 

 TTT = 8 '68 Ibs. per sq. in. In other words, divide 



Tank. 

 FIG. 50. 



the head by a constant = 2*3 and the result will be 

 the pressure in Ibs. per sq. in. 



Another point of interest in the use of gravitating 

 water mains is the use of a syphon, and by means of 

 which in certain cases a supply can be drawn from a 

 well (not over 28 ft. in depth to lowest water level) 

 without pumping or other mechanical means. Every 

 advantage should, of course, be taken of cases of this 

 sort. Referring to fig. 50, we have a well on a hill- 

 side, required to deliver into a tank as shown with- 

 out pumping up from the well. Now everyone is 

 well acquainted with the mercurial barometer, in 

 which a column of mercury will stand in a vacuum 



