ally in many localities, and that occasionally higher pressures are
recorded in exposed positions. Thus at Bidstone, Liverpool, where the gauge
has an exceptional exposure, a pressure of 80 lb per sq. ft. has been
observed. In tornadoes, such as that at St Louis in 1896, it has been
calculated, from the stability of structures overturned, that pressures of
45 to 90 lb per sq. ft. must have been reached. As to anemometer pressures,
it should be observed that the recorded pressure is made up of a positive
front and negative (vacuum) back pressure, but in structures the latter
must be absent or only partially developed. Great difference of opinion
exists as to whether on large surfaces the average pressure per sq. ft. is
as great as on small surfaces, such as anemometer plates. The experiments
of Sir B. Baker at the Forth bridge showed that on a surface 30 ft. x 15
ft. the intensity of pressure was less than on a similarly exposed
anemometer plate. In the case of bridges there is the further difficulty
that some surfaces partially [v.04 p.0549] shield other surfaces; one
girder, for instance, shields the girder behind it (see _Brit. Assoc.
Report_, 1884). In 1881 a committee of the Board of Trade decided that the
maximum wind pressure on a vertical surface in Great Britain should be
assumed in designing structures to be 56 lb per sq. ft. For a plate girder
bridge of less height than the train, the wind is to be taken to act on a
surface equal to the projected area of one girder and the exposed part of a
train covering the bridge. In the case of braced girder bridges, the wind
pressure is taken as acting on a continuous surface extending from the
rails to the top of the carriages, plus the vertical projected area of so
much of one girder as is exposed above the train or below the rails. In
addition, an allowance is made for pressure on the leeward girder according
to a scale. The committee recommended that a factor of safety of 4 should
be taken for wind stresses. For safety against overturning they considered
a factor of 2 sufficient. In the case of bridges not subject to Board of
Trade inspection, the allowance for wind pressure varies in different
cases. C. Shaler Smith allows 300 lb per ft. run for the pressure on the
side of a train, and in addition 30 lb per sq. ft. on twice the vertical
projected area of one girder, treating the pressure on the train as a
travelling load. In the case of bridges of less than 50 ft. span h