226 Practical Remarks on Hydrometry. 
use, might have originally been more readily derived from 
the actual experiments than from empirical formule. 
Some years ago I computed for English inches, for my 
own use, the following short table of ratios between surface 
and mean velocities, and which ratios give results conform- 
able to De Prony’s Rule. 
Surface Velocities Multipliers for 
in Inches. Mean Velocities. 
to 3 dH aan 0-75 
3 to 8 ae Tak 0°76 
8 to 13 ae hae 0:77 
13 to 18 Ba ae 0:78 
18 to 25 ood eee 0-79 
25 to 85 eae Ang 0°80 
35 to 45 aan skls 0°81 
45 to 55 oye Ke 0°82 
55 to 65 ae ae 0°83 
65 to 76 rd Wes 0°84, 
76 to 87 ae ae 0°85 
87 to 100 bao ay 0°86 
When I have employed a float for determining surface 
velocities, I have taken the following precautions to ensure 
due accuracy. Having chosen for the measurement of the 
discharge, a site where the apparent regularity of the width 
of the stream, and general aspect of its banks, led me to 
suppose that no very material variation of the cross profiles 
of the bed of the stream would occur for a distance of fifty 
feet, I then departed from the usual mode of procedure inas- 
much as I took eight or ten cross sections within the longitu- 
dinal extent of fifty feet. 
If these cross sections and corresponding sectional areas 
did not differ to any great extent, they afforded a proof that 
the site was favourable for the correct determination of the 
discharge of the stream. The mean of the sectional areas 
was then taken as the mean transverse sectional area, corres- 
ponding to the assumed longitudinal distance of fifty feet, 
which distance was defined by parallel transverse lines per- 
pendicular to the axis of stream, and indicated on the banks 
of the stream by ranging rods. 
Bearing in mind the fact that a large floating body, through 
not participating in the irregular intimate motion of the 
particles of water of a running stream, would move with 
somewhat greater velocity than a minute fragment of the 
same body, I employed a very small float, consisting of a very 
diminutive vial, so weighted with sand as to cause it to float 
along when corked, with the top of the cork flush with the 
surface of the water. 
