116 
Proceedings of the Royal Society of Edinburgh. [Sess. 
of their moving parts and consequent “ lag.” In each class instances are 
given of instruments which are free from the error, but the Robinson cup 
anemometer — in which the authors are more particularly interested — is 
not one of those. As nothing was known definitely as to the effects of 
lag in this case, an investigation was undertaken, using a specially 
designed apparatus analogous to the cup anemometer, and, for greater 
accuracy of measurement, currents of water instead of air. 
A description of this apparatus is given; and the remainder of the 
paper is occupied with the mathematical theory and the results of experi- 
ment. The most striking of these is the fact that the instrumental error 
is always positive; that is to say, that when the current fluctuates the 
instrument registers too great a flow. 
The authors desire to record their thanks to Professor MacGregor for 
the facilities he has provided, and for the kind interest he has taken in 
their work. 
Part of the special apparatus described was obtained by a grant from 
the Tait Memorial Fund. 
Physics Department, 
.Edinburgh University. 
(. Issued separately April 11 , 1913 .) 
