22 BULLETIN 376^ U. S. DEPAETMENT OF AGEICULTUEE. 



opened. This process was repeated every 10 minutes. All other 

 readings were taken by alternately reading high and low gauges 

 with the mercury just ahve, the corresponding reading for the other 

 gauge being computed from the dead readings as described above. 

 Since the only change in the total length of mercury thread was 

 due to temperature changes, and since the gauges, which were made 

 of the highest grade of manometer tubing, were practically uniform 

 in diameter, no error was introduced by reading but one leg at a 

 time, alternately. (See p. 93.) 



DETERMINATION OF LOST HEAD. 



The exact amount of hf (fig. 1) must be determined. Where a 

 water column is used, say at gauge No. 2, the elevation Ej is the 

 gauge reading added to the elevation of the gauge zero above an 

 assimied datum, with proper corrections (see p. 23). Where a 

 mercury manometer of the U-tube pattern is used, the reasoning is 

 as follows: It is desired to know the elevation E^ (fig. 1) for a water 

 column which is the equivalent of a mercury column in a U-tube 

 placed as for gauge No. 1. Referring to figure 2, the mercury in 

 the two legs of the U-tube below c-d will be seen to balance. There- 

 fore the pressure of the water at c is just balanced by the column of 

 mercury X. But the pressure at c equals that at d. If the mercury 

 X were replaced with water it would reach an elevation sX above 

 d, where s is the specific gravity of the particular mercury in the 

 gauge, compared with the particular water in the pipe. But the 

 elevation to which this water column would reach is the desired 

 elevation, E^. Therefore the elevation Ei=sX + y above the as- 

 sumed datum. As apphed to these experiments, referring to figures 

 1 and 2, the difference in elevation between the readings of the low 

 gauge and the high gauge multipHed by the specific gravity of the 

 mercury and added to the elevation of the low-gauge reading gave 

 the elevation of the equivalent water column when the proper cor- 

 rections had been appUed. 



— > CORRECTIONS. 



Although quite numerous, the principles involved in all of the 

 necessary corrections have been the subjects of such thorough inves- 

 tigation that appreciable errors are not liable to result from their use. 



Temperature. — Corrections are necessary for the temperature 

 changes in both air and water. A temperature of 15° C. was adopted 

 as standard and the specific gravity of the mercury used in the tests 

 was referred to that temperature, being compared to distilled water 

 at the same temperature. 



The mercury column balances the pressure of the water in the 

 pipe, but this water may be either heavier or lighter than distilled 



