58 BULLETIN 118, U. S. DEPARTMENT OF AGEICULTUBE. 



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. 



]\Iultipl3' the area of the cross section by the velocity of the stream 

 in feet per minute, and the result vrill be within 12 per cent of the 

 discharge in cubic feet per minute. 



To compute the capa€ity of pumping engin-es. — 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 by 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," llxV pounds, at 62° F. is equal to a column of 

 mercmy 29.9 inches high, or a column of water 33 j% 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 7^ 

 pounds ordinary coal, or about 1 pound of coal or 2^^ 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, bj^ 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 



