24 BULLETIN 376, U. S. DEPAETMENT OF AGEICULTUEE. 
for four injections of color in a pipe carrying up to 60 second-feet, 
while in the 13J-foot pipe (No. 52) the total contents of the " fluor- 
escein gun" (fig. 3) were injected at one time, and for the volume 
of water carried in this pipe (the maximum was 871 second-feet) a 
saturate solution was used. Though not measured, this consisted 
of about 4 teaspoonsful of the powder for each "shot" of about one- 
third pint. The powder dissolved readily in cold water. 
In making a test the coupling W is opened and the solution poured 
into the pressure tube X. The gun is again connected with the 
apparatus by the coupling W. With E closed V is opened. Pressure 
in the wood pipe enters the gun, making pressures in both gun and 
pipe equal. In order to inject the color into the pipe the only thing 
necessary is to increase the now existing pressure in the gun. After 
V has been closed the gun is pumped up like a bicycle tire. While 
noting the time to a second the operator opens the cock V. By 
the hissing sound, it is probable that the jet passes well across the 
diameter of a medium-sized pipe. If the contents of the gun are to 
cover three or four injections V is opened and almost immediately 
closed. If all the contents are to be used a few quick strokes of the 
pump, after V has been opened, will clear the gun in a very few 
seconds, the mean time being accepted in later computations. 
The observer at the outlet, provided with a watch agreeing to the 
second with that used in timing the start of the color, notes to the 
second the first and last appearance of the color. The color is ex- 
tended by the variation in the velocity throughout the section of 
the pipe. This extension covers about 8 per cent of the total time 
the color spends in the pipe. Comparison with carefully constructed 
weirs shows that the color method is correct within about 3 per cent. 
Wherever possible, a comparison between color and current meter was 
also made. To secure comparative results the time the color spent 
in the pipe is taken as from the moment of injection to the mean 
between first and last sight at the outlet. These comparative tests 
are shown in Table 1. 
