THE PLOW OF WATER IN WOOD-STAVE PIPE. 23 



water. Hydrometer tests of tlie water at the time of the experiment 

 showed the specific gravity of the water for that temperature. A 

 table was computed showing the proper specific gravity factor to 

 apply to convert the mercury column to the equivalent water column 

 for any observed specific gravity of water.^ No additional correc- 

 tion is necessary for the temperature of the water as the hydrometer 

 takes this into consideration. 



The pressure in the pipe (fig. 2) supports the mercury column X 

 and in addition the water column from the pipe to the elevation 

 of c. If this water is of a different temperature from that in the pipe 

 a correction is necessary, but in these experiments the water was 

 kept at about the same temperature by frequently blowing off the 

 water in the rubber pressure tube. The length of this water column 

 in a mercury gauge at no time was more than 1 or 2 feet. 



However, in a water column manometer the difference in tempera- 

 ture must be considered. The temperature of the water in the tube 

 was taken as that of the air adjoining, while the temperature of the 

 water in the pipe was determined at the same time that its specific 

 gravity was tested. Water columns were not blown off but air 

 bubbles were driven to the glass tube by striking the rubber tubing 

 sharply with a stick. Siphons in the pressure tubing were carefully 

 prevented. 



Capillarity. — Water rises by capillarity in a small tube and mercury 

 is depressed. Two sets of glass tubes were used for water columns. 

 For one, with inside diameter of 4.5 mm., water rises 0.017 foot, 

 while in the other set, with diameter of 5.6 mm., the water rises 0.01 

 foot. 



MEASUREMENT OF MEAN VELOCITY. 



As a rule, each pipe tested presented its own problem as to the 

 method to be adopted to determine the mean velocity of the water, 

 and in case this method digressed from one of the following standard 

 methods it is described. 



Current meter. — Where the water entered or left the pipe in an 

 open channel the discharge was determined with a current meter, 

 and the velocity in the pipe was secured by dividing this discharge 

 by the area of the pipe. The two-tenths and eight-tenths depth 

 method was used, as the results obtained in this way, when com- 

 pared with the discharge found by the multiple-point method, gen- 

 erally agree with it to about 1 per cent. 



Fluorescein. — About 1 teaspoonful of fluorescein (in the form of 

 red powder) dissolved in about a pint of water gave sufficient solution 



1 The mercury used in experiments conducted by the writer was tested for specific gravity in the labo- 

 ratory of Nutrition Investigations, U. S. Department of Agriculture. The specirx gravity was found to 

 be 13.575 at 15° C, compared with distilled water at 15° C. These were the temperatures adopted as basic 

 for the computation of results. 



