METEOROLOGY. 197 
then, above surrounding objects, is absolutely necessary. Self-registering 
weathercocks, called anemometers, or anemographs, have also been con- 
structed by Landriani, Parrot. Traill, Osler, and others, where the direction 
of the wind during the twenty-four hours is traced directly on paper without 
the necessity of an attendant. 
Besides the direction of the wind it is necessary to consider its velocity 
and strength. The simplest method of ascertaining the velocity of wind is 
to make use of an apparatus similar to that employed in ascertaining the 
velocity of a current of water. Thus, a light body, as a piece of paper or a 
downy feather, &c., may be let loose into the air, and the distance traversed 
in a given time noted ; this is, however, entirely inapplicable in high winds, 
owing to the irregularity of the motion, and the difficulty of properly 
regulating the experiment. Some have endeavored to use the passage of 
the shadows of ‘clouds over the surface of the earth to ascertain this velocity, 
but this method is not very applicable, owing to the uncertainty of the path 
and the small portion measurable by a single observer. The most satisfac- 
tory and generally used method is to ascertain the perpendicular pressure 
of the current against an opposed surface of known area, assuming that the 
force of the wind is a consequence or a function of its velocity. Con- 
trivances for measuring the force of the wind are called anemometers, in the 
restricted sense of the term. They have been proposed and constructed in 
great variety. The first method proposed was to allow the wind to strike 
against a vertically depending disk or plate. and to determine its force by 
the angle through which the plate was lifted. This idea was the basis of the 
anemometer of Pickering and Oertel. In Dalberg’s construction, the disk, 
instead of depending, was erected vertically on a hinge, and kept to the 
wind bya great vane. More recently, Parrot has proposed instead of a disk 
to take a hollow ball, which, suspended freely by a rod, shall be raised by 
the wind, the angle of elevation being measured on an attached quadrant, 
and from this angle the force of the wind determined. One of the oldest 
and simplest anemometers is that of Bouguer, consisting of a square plate 
fastened to a four-sided rod. This rod fitted in a hollow four-cornered 
parallelopipedon, and pressed against a spiral spring contained in it. The 
plate being made to face the wind, the amount of pressure exerted on the 
spring was measured by an index. This instrument has recently been much 
improved by Beaufoy. 
In another class of anemometers the wind turns wheels or vanes: to these 
belong those of Christiani, Wolf, Leutmann, and others, the ingenious 
self registering anemometer of Michael Lomonosow, and the hydrometric 
vane of Waltmann. By means of this latter instrument the velocity of run- 
ning water may be ascertained as well as that of the wind. It consists ofa 
small windmill, to whose axis an endless screw is attached, which catches 
in the teeth of a wheel ; the axis of the latter is provided with an index for 
the purpose of measuring the entire number of revolutions of the mill. 
Among the more recent instruments the portable one of Lind (probably 
invented by Hales) deserves special attention, on account of the small 
surface presented to the wind, large surfaces always involving great 
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