■206 
THE METHODS OF WIND MEASUEEMENT. 
means of a Royal Society grants conducted a series of very careful 
experiments, in order to determine tlie constants in an expression 
which his investigations led him to conclude would hold between 
V and V, viz. : 
a — 2 i3Yv — — F = 0 . 
where F is the friction of the instrument and may be measured, 
and a, 13, and 7 have to be found ; but the result was admitted 
by him to be anything but satisfactory/ the values for 7 being so 
discordant as +21-554, —59*672, —3-617 and +9*814. This 
series of experiments, however, and also later ones, have shewn 
that at tolerably high velocities 
Y = 2‘5v nearly. 
But the relation really alters for every change of velocity and 
size of the cups. 
There is not space to describe here the various modifications 
of cup anemometers. The simplest and most general mode of 
registering the number of revolutions of the cups is by means of 
a train of wheels, the last one or two of which are graduated as 
in a gas or water meter. But these are not truly self-recording, 
and their motion is assumed to be uniform during the intervals 
at the beginning and end of which they are observed. 
Some instruments communicate the revolutions of the cups 
above to the observing rooms beneath by means of an electric 
current, in which case electric contact is made and broken every 
10 or 100 revolutions, as in an instrument devised by Professor 
Barrett, of Dublin, The author has devised a mode of graphically 
representing the velocity thus obtained by causing marks to be 
made at each interruption of the circuit upon a strip of paper 
moved by clockwork. At equal distances along this strip 
ordinates are drawn, whose height is proportional to the number 
of marks in the adjacent distance or interval, and the ordinate at 
I Phil. Trans. Royal Soc., Part II., 1878, p. 807. 
