226 



Practical Remarks on Ilydrometry. 



use, might have originally been more readily derived from 

 the actual experiments than from empirical formulae. 



Some years ago I computed for English inches, for my 

 own use, the foUoAving 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 Indies. Mean Velocities. 



1 to 3 0-75 



3 to 8 ... ... 0-76 



8 to 13 ... ... 0-77 



13 to 18 0-78 



18 to 25 0-79 



25 to 35 0-80 



35 to 45 0-81 



45 to 55 0-82 



55 to 65 0-83 



65 to 76 0-84 



76 to 87 0-85 



87 to 100 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 favotirable 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 

 sam« 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. 



