36 MANUAL OF CURRENT OBSERVATIONS 
Free Floats 
90. In some narrow passages where the current is swift, it may be impracticable 
to anchor a boat to observe the currents by the usual means. A method of securing 
observations in such cases is to establish two parallel ranges on shore at approximate 
right angles to the stream, measuring and recording the distance between the ranges. 
Floats are then thrown into the stream at regular intervals and allowed to drift by the 
ranges while observers equipped with stop watches and stationed at the ranges observe 
the time of passage from one range to the other. 
91. Two types of floats have been used, depending upon whether they are thrown 
from the shore or dropped from a boat at some distance from shore. In the first case, 
strips of 2-inch by 2-inch lumber about 18 mches long may be used advantageously. 
Their visibility is increased by wrapping them in cloth. If the float is to be dropped 
at a considerable distance from shore, something larger will be necessary. Such a 
float may consist of two horizontal crosspieces about 3 feet im length and an upright 
about 4 feet long, the material being 2-inch by 2-inch lumber. A small piece of cloth 
attached to the top will aid the visibility, but should not be so large as to cause the 
movement of the float to be materially affected by the wind. 
92. The velocity of the current may be determined from the distance between 
the ranges and the time necessary for the float to travel this distance by means of the 
following formula: 
distance between ranges in feet 
time to travel distance in seconds 
Velocity in knots X0.592 
When the same ranges are used for a large number of observations, the computation 
of the velocities will be facilitated by a table prepared from the above formula. 
