PKESSURE GAUGES 



19 



pressure is found in the transmission of the pressure of the atmosphere 

 on the surface of water to every part of its depth. Thus the pressure 

 intensity at any depth h is strictly that due to the weight of the super- 

 posed water, together with the atmospheric pressure. 



Atmospheric Pressure. The pressure of the atmosphere varies from 

 day to day at the same place, and from place to place at the same time- 

 Its mean value at sea level 

 is about equivalent to that 

 at the base of a mercury 

 column 30 inches in 

 height, or 14'7 Ibs. per 

 square inch. This is 

 equivalent to the pressure 

 at the base of a water 

 column of approximately 

 84 feet in height, and this 

 is usually taken as the 

 height of the water baro- 

 meter. Thus the true 

 pressure intensity at a 

 depth h below the free 

 surface of water is given by 

 62-4 (h + 34) Ibs. per 

 square foot. 



In most hydrostatic 

 problems, however, it is the 

 pressure in excess of that 

 due to the atmosphere 

 which is required, so that 

 the pressure at a submerged point is commonly taken as being that due 

 to the head of water alone. 



ART. 9. PRESSURE GAUGES. 



The most accurate method of measuring the difference between the 

 pressure at any point in the length of a pipe and that of the atmosphere, 

 is by means of a piezometer or manometer in which the difference is 

 measured by the height of a column of liquid supported by the excess 

 pressure. Where the difference of pressure is not more than one or two 

 pounds per square inch, this liquid may conveniently be water, when the 

 pressure difference is given by k -j- 2*31 or '433 h Ibs. per square inch, 



c2 



FlG. 7. Bramah's Press. 



