%\it ij\m^ of twtara anb 
Slworbiirg apparatus. 
By Peofessor H. S. HELE SHAW. 
HE author has for some considerable time been occupied, 
X during intervals of leisure, in designing apparatus for 
measuring velocity, such as anemometers, current meters, etc., 
and contributed, in 1882, a paper to the Institution of Civil 
Engineers, in which an attempt was made to treat the question 
in a systematic manner. It was there shewn that tliere are two 
modes of measuring velocity, one or the other of which are 
employed with all instruments for this purpose. They are — 
(1) The conjoint observation of time and space, the ratio of 
which, if uniform, or of the increments of w'hich, if not uniform, 
is velocity. 
(2) The direct observation of tlie effect due to velocity. 
It is often of great importance to see at a glance what the 
velocity is, and for this purpose the second of these modes is 
almost invariably employed. Cases of its use are seen in 
pressure anemometers, the d’Arcy gauge, and Pitot tube, the 
current meter of M. Perodil, and, notably, in a class of speed 
indicators, of which there are numerous recent varieties, which 
depend for their action upon the position of a revolving weight 
■or weights. 
When the velocity of a moving body is required to be known 
at a distance, this method has serious objections. Communica- 
tion is either made by the varying pressure of a column of fluid, 
