THE FLOW OP WATER IN" WOOD-STAVE PIPE. 23 
water. Hydrometer tests of the 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. 1 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 
i 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 specific 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. 
