196 PHYSICS. 
and again ascended more than five lines in three quarters of an hour. From 
Humboldt’s observations the hurricanes between the tropics are not 
accompanied by so great a depression of the barometer as is generally 
supposed. | 
4. Of the Winds. 
All motions of the atmosphere, or aerial currents, are known by the 
general term winds. These almost always arise from an unequal heating of 
the earth’s surface and of the incumbent air, although other causes may 
occasionally operate, as, for instance, the sudden condensation of watery 
vapor in the atmosphere. The theory of winds is well illustrated by an 
experiment as figured in pl, 23, fig. 62. If, during winter, the door of a 
warm room leading into an apartment or passage-way that is not heated, be 
slightly opened, and a burning candle held to the upper end of the crevice, 
the flame will be driven outwards, and thereby demonstrate the existence 
of a current of heated air from the upper part of the room. About the 
middle of the opening the flame will be vertical, owing to the absence of both 
an inward and outward current. At the bottom, again, a current will be 
again sensible, consisting of the colder external air pressing into the room to 
supply the loss occasioned by the upper outward current. This air then, 
atter entering the room, becoming heated, and therefore lighter, ascends and 
passes out at the top again. Precisely in the same manner the air in the 
warmer regions of the earth ascends and flows over the colder, while that 
from the surrounding colder regions flows in from below. 
To determine the direction of winds at the surface of the earth, weather- 
cocks or wind-vanes are used. These consist of a flat thin piece of sheet 
iron or other material, of an appropriate shape. This is placed in a vertical 
plane, and turning on a vertical axis passing through its centre of gravity. 
The surface of the vane on one side of this axis must be greater than on the 
other, so that when struck by the wind it may have a determinate direction. 
Thus we may construct the vane in the shape of an arrow broadly feathered ; 
the barb will then point in the direction from which the wind comes. This 
direction is generally estimated by the eye, but may be obtained more 
accurately by fixing under the vane a series of rods, connected in a frame- 
work, and pointing to the four quarters of the horizon. Each rod has a 
letter at its extremity indicating this direction as N., S., E.. W.; sometimes 
there are four others to indicate the N.W., S.W., N.E., 8.E. points of the 
compass. A very convenient construction of the vane is to have its axis 
passing down through the roof into a chamber. On the lower end of 
the axis is to be fixed an index, which shall rotate in a circle marked 
with the directions of the wind. This index, turning simultaneously and 
equally with the vane, will always tell the direction of the wind without the 
necessity of going outside of the house. It should have been before 
mentioned, that vanes, to be of scientific value, must be as much as possible 
free from the minor currents produced by local obstructions ; their elevation, 
370 
