204 
THE METHODS OP WIND MEASUEEMENT. 
two classes of instruments, and those directly measuring that 
quantity have to a great extent superseded the other class ; 
hut even the engineer, who generally requires to know the 
pressure of wind in its effect on structures, has to consider two 
objections to the use of the pressure anemometer — 
( 1 st) From the way in which the wind moves, 
(2nd) From the nature of wind pressure itself. 
If the wind blew steadily on any area the pressure, at any rate 
on that area, might be correctly given by the resistance of the 
springs, but this is well known to be the opposite of what occurs ; 
the wind varies in intensity from moment to moment, to a 
considerable degree even in mid ocean ; whilst on land it is, 
during a gale, in a state of continual oscillation, and its action 
is rather that of an impulsive blow. The result is a record in 
excess of a mean pressure, followed by a recoil of the springs, 
as appears to be shewn by the readings of Osier’s anemo- 
meter, aud indeed it does not appear how an accurate mean 
pressure is to be determined from such a curve. Now a structure 
differs in a material way from the pressure board of an anemo- 
meter, for though a steady pressure must affect each equally, 
not so a gust of wind ; the two results are totally different, 
and they must not be confused. The maximum effect of a gust of 
wind in a pressure anemometer does not give any certain idea of 
the pressure really taking effect on a structure, unless it is 
known that the gust was of sufficient duration to be felt as a 
steady pressure, and it is conceivable that some of the conflicting 
views Vhich exist may result from the effect of a gust on a 
pressure board being quoted, without the duration of the 
gust being known. Again, as already mentioned in the memo- 
randum of Prof. Stokes and Sir W. Armstrong, the relative 
pressure over two areas of different amount is quite unknown, 
that is to say, even knowing the pressure on one square foot it is 
impossible to say what at the same place and time the 
