the ordinary pressure plate is admirably adapted for measuring the
force of a steady and uniform wind, it is entirely unsuitable for
following the rapid fluctuations of the natural wind. To make
matters worse, the pen which records the motion of the plate is often
connected with it by an extensive system of chains and levers. A
violent gust strikes the plate, which is driven back and carried by
its own momentum far past the position in which a steady wind of the
same force would place it; by the time the motion has reached the pen
it has been greatly exaggerated by the springiness of the connexion,
and not only is the plate itself driven too far back, but also its
position is wrongly recorded by the pen; the combined errors act
the same way, and more than double the real maximum pressure may be
indicated on the chart.

A modification of the ordinary pressure-plate has recently been
designed. In this arrangement a catch is provided so that the plate
being once driven back by the wind cannot return until released by
hand; but the catch does not prevent the plate being driven back
farther by a gust stronger than the last one that moved it. Examples
of these plates are erected on the west coast of England, where in the
winter fierce gales often occur; a pressure of 30 lb per sq. ft. has
not been shown by them, and instances exceeding 20 lb are extremely
rare.

Many other modifications have been used and suggested. Probably a
sphere would prove most useful for a pressure anemometer, since owing
to its symmetrical shape it would not require a weathercock. A small
light sphere hanging from the end of 30 or 40 ft. of fine sewing
cotton has been employed to measure the wind velocity passing over
a kite, the tension of the cotton being recorded, and this plan has