Hydrometrical Observations. 383 
ance in the motion of the stream, and alters the velocity of the current, 
so that the result indicated by the clapsed time is more or less vitiated, 
and the mean velocity deduced from such data, is not, in almost any case, 
that which exists at the line of cross section. It is also impossible, by 
this method, to obtain a sufficient number of distinct and independent 
observations, applicable to each division of the stream, as the eddies and 
irregularities of the current which exist in all rivers, generally cause the 
lines passed over by the floats to cross and interfere with each other in 
such a manner as to destroy all connection between any given series of 
observations, and the several compartments of the river, whose mean 
velocity they were intended to ascertain. 
The superiority of the method which I am about to describe, consists 
in ascertaining the velocity of each portion of the stream, in the exact 
line in which the cross sectional area is taken. The instrument em- 
ployed for this purpose is a modification of the tachometer of Woltmann, 
which is in general use in France and Germany, both as an anemometer 
and a hydrometer, being made of the degree of delicacy suited to the 
purpose to which it is to be applied. In this instrument the velocity is 
measured by the current impinging on a vane and causing it to revolve, 
the number of revolutions made by the vane being registered on an in- 
dex, which is acted on by a set of toothed wheels. 
The construction of this beautiful instrument, and the manner in which 
it acts, will be best described by a reference to the accompanying cut, 
fig. 7., which is taken from 
MAM 
