Included in the following matter, arranged by the editor of this journal, are only the fundamentals of hydraulics as adapted to fire department usage. The methods outlined are recognized by the Civil Sendee authorities as satisfactory for use in fire department examinations in New York City.

Fig. 128

Principles of Pressures in Fluids

The six fudamental principles underlying the theory of pressure in fluids may be stated as follows:

  1. Fluid pressure is perpendicular to any surface on which it acts.
  2. Fluid pressure at a point in a fluid at rest is of the same intensity in all directions.
  3. Pressure applied to a confined fluid from without is transmitted in all directions without diminution.
  4. The downward pressure of a liquid in an open vessel is proportional to its depth.
  5. The downward pressure of a liquid in an open vessel is proportional to its density,
  6. The downward pressure of a liquid on the bottom of a vessel is independent of the shape of the vessel itself.

Water Pressure

As the most common unit used in the measurement of water is the gallon, a few words regarding its relation to the other units will not be out of place here. There are 231 cubic inches in a U. S. gallon. In a cubic foot there are 12x12x12—1,728 cubic inches. Hence in a cubic foot there are 1728-f-231, or 7.481 gallons. A cubic foot of water of average quality weighs 62.5 pounds. The weight of a gallon is then 62.5/7.481 (the number of gallons in a cubic foot), or 8.35 pounds.

Pressure Units

Pressure is usually stated in pounds per square inch. Where static pressure only is encountered it can readily be determined if the head is known, To find the pressure where the head in feet is known it is only necessary to multiply the height of the water column in feet by .434 pounds, the pressure exerted by a column of water one inch square and one foot high, and the pressure in pounds per square inch results. For example, suppose there is a tank on a building and the water in it is just 100 feet above the street level. The pressure at the street level would be 100X-434—43.4 pounds per square inch.

A column of water one square inch in area and one foot high produces a pressure of .434 pounds; in other words, water one foot deep exerts a presure of .434 pounds per square inch. To find the depth of water, or head, required to produce 1 pound pressure divide 1 by .434, and the result is a head of 2.304 feet. To sum up, to find the height of the water column when only the pressure it exerts is known, multiply the pressure in pounds per square inch by 2.304, the height of a 1 square inch column of water producing 1 pound pressure, and the answer is the required height.


Suction may be termed the reverse of pressure, the same as pulling, the reverse of pushing. But the term suction is misleading. The conception of sucking water as the act of pulling or drawing water is erroneous; for to draw or pull an object prescribes the necessity of having some form of connection to it. In the case of sucking up water, what is really done is to reduce the air pressure in an airtight vessel in direct communication with a source of water supply by removing the contained air, the atmospheric pressure outside forcing the water into the vessel.

For ordinary use the average atmospheric pressure is taken at 14.7 pounds per square inch. This pressure is capable of sustaining a column of water 33.9 feet in height, or a column of mercury 29.9 inches in height, under a perfect vacuum.

Effect of Atmospheric Pressure on Pumps

Fire engines when in good condition can lift water a vertical distance of about 25 feet at sea level. In other words, they can reduce the air pressure above the water to such an extent that the 14.7 pounds of atmospheric pressure forces the water up vertically a distance of 25 feet. If, however, the engine were stationed two miles above sea level, the atmospheric pressure would be only 9.8 pounds and the same pumps would only be capable of lifting water about 17 feet.

(To be Continued)

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