however, that, although over a period of one hour the wind may
_average_ this velocity or pressure, it will vary considerably
from moment to moment, being far in excess at one time, and
practically calm at another. The velocity of the wind is
usually taken by a cup anemometer having four 9 in cups on arms
2 ft long. The factor for reducing the records varies from 2 to
3, according to the friction and lubrication, the average being
2.2.

The pressure is obtained by multiplying the Beaufort number
cubed by 0.0105; and the velocity is found by multiplying the
square root of the Beaufort number cubed by 1.87.

A tidal wave will traverse the open sea in a straight line, but
as it passes along the coast the progress of the line nearest
the shore is retarded while the centre part continues at the
same velocity, so that on plan the wave assumes a convex shape
and the branch waves reaching the shore form an acute angle
with the coast line.




CHAPTER III.

CURRENT OBSERVATIONS.


There is considerable diversity in the design of floats
employed in current observations, dependant to some extent upon