46 AN AUTOMATIC HOUSEHOLD FILTER 
a small pipe is let in at the top bend of the swing arm to 
admit air. By this means a rise of level of water inside 
the filter lowers the outlet, so that each inch increase of rise 
in water level gives virtually two inches more pressure 
on the filter, thus practically doubling the head of water 
that can be obtained in the filter. The fall of the swing 
arm also calls attention to the fact that the filter is choking, 
but the fall is so gradual that a month can elapse after the 
arm starts to fall ere cleaning is necessary. Before the 
filtered water pipe enters the storage tank, there is a draw- 
off pipe with cock connected to the waste discharge, so that 
the filtered water can be run to waste for two days after 
cleaning off the top sand, and thus save the contamination 
of the clean water already in the storage tank. As the 
filtered water is always found to be absolutely devoid 
of oxygen, it is run over an aerator before going into the 
storage tank. The stored filtered water in the tank 1s 
always saturated with oxygen. The pipe from the water 
main passes close up beside the storage tank, and a floating 
ball connected through a slot in the tank with a ball cock 
on the pipe, controls the supply to the filter. The drawing 
off of water from the storage tank lowers the ball, opens 
the cock, and water is delivered on the filter until the filtered 
water again rises sufficiently to close the cock. To regulate 
the supply to the filter, a small cistern, just large enough 
for a ball cock, is placed over the filter. In our case, nine 
inches of water was the depth obtained in this cistern. 
In the bottom of this cistern there is a standard orifice, 
with about quarter of an inch of pipe around it to ensure 
the water dropping straight down and not running along 
the under side of the cistern. The diameter of the orifice 
is so adjusted that with the head of water obtainable in 
the cistern, it delivers water on to the filter at the standard 
rate of three million gallons per acre of sand surface per 
twenty-four hours, and cannot possibly deliver faster than 
this. As the ball cock in the cistern of course delivers. 
at a much greater rate than this, the cistern is nearly always 
full when the filter is working. The water delivered from 
the main is practically devoid of oxygen, so we erate the 
water by running it down a ripple on to the side of the tank, 
and find by that means that the water on top of the filter 
