58 BULLETIN 718, U. S. DEPARTMENT OF AGRICULTURE. 
than the bottom or sides the average velocity being approximately 
83 per cent of the surface velocity at the middle, it is recommended to 
measure a distance of 120 feet for the float to run and reckon it 
as 100. 
Multiply the area of the cross section by the velocity of the stream 
in feet per minute, and the result will be within 12 per cent of the 
discharge in cubic feet per minute. 
To compute the capacity of pumping engines. — Multiply the area 
of the water piston in inches by the distance it travels in inches in a 
given time. Deduct 3 per cent for slip and rod displacement. The 
product divided b}^ 231 gives the number of gallons in the time 
named. 
Barometric pressure varies at different altitudes with an equiva- 
lent variation in the vertical suction lift of the pumps. 
An " atmosphere," 14 T 7 F pounds, at 62° F. is equal to a column of 
mercury 29.9 inches high, or a column of water 33 r % feet high. 
Water at high temperature can not be raised any considerable 
distance by suction, as the vapor discharged from the water follows 
the receding piston of the pump and resists the entrance of the 
water; consequently, to pump hot water always have the supply 
above the pump so that it will be supplied from a head. 
STEAM. 
Under the ordinary atmospheric pressure of 14.7 pounds per square 
inch, water boils at 212° F., passing off as steam, the temperature 
at which it boils varying with the variation in the pressure. Steam 
rising from water at its boiling point (212° F.) has a pressure equal 
to the atmosphere, viz, 14.7 pounds to the square inch at sea level. 
To evaporate 1 cubic foot of water requires the consumption of 7J 
pounds ordinary coal, or about 1 pound of coal or 2J- pounds wood 
to 1 gallon of water. Steam is transparent, colorless, dry, and 
invisible except when partly condensed, when the mist makes it 
visible. Dry steam does not contain any free moisture; wet steam 
contains free moisture in the form of spray or mist and has the same 
temperature as dry saturated steam of the same pressure. Saturated 
steam is steam in its normal state, that is, steam whose temperature 
is that due its pressure, by which is meant steam at the same tem- 
perature as that of the water from which it was generated and upon 
which it rests. Superheated steam is steam at a temperature above 
that due its pressure after leaving the liquid from which it is 
generated. 
One cubic inch of water evaporated into steam at 212° F. becomes 
1,645 cubic inches, that is, nearly 1 cubic foot. Water introduced 
into superheated steam Will be vaporized until the steam becomes 
saturated and its temperature becomes that due its pressure. Cold 
