ANEROID BAROMETER IN GEOLOGICAL SURVEYING. 131 
its apparent weight, and causes the barometer to fall. A falling 
barometer indicates an approaching cyclone. Yhe fall continues 
until the trough, or line perpendicular to the direction in which 
the cyclone is moving, is reached, when it is changed to an 
upward movement. 
This upward impulse of the air particles results in a loss of 
heat, and condensation of the contained moisture into visible 
drops, which are blown by the rapid currents of the upper atmos- 
phere into long, wispy, hairy, cirrus clouds, which reach far 
beyond the cyclone’s front. Gradually thickening cirrus clouds 
herald a cyclone. Behind the axis, the outflowing air has to con- 
tend against the general motion of the atmosphere, which is from 
west to east, and hence its velocity is much lessened. Any 
moisture condensed in this portion of the area will tend to form 
lumpy cumulus, rather than windy cirrus clouds. Cumulus clouds 
mark the rear of the cyclone. 
The intensity of a cyclone, or the velocity of the ascending 
current, and consequently the rate at which the barometer rises 
or falls (barometric gradient) depends largely on the supply of 
moisture. If the amount of moisture carried into the area 
remains constant, the intensity of the cyclone, and the gradient, 
will be constant. If it increase, the intensity and gradient will 
increase, and if it diminishes, both will diminish, or if it fails, 
both will disappear. If the supply comes equally from all 
directions, the cyclone will remain nearly or quite stationary, 
but if the winds from one quarter bring more moisture than 
those from the others, the cyclone will move in that direction 
with a velocity governed by the supply. Changes in direction 
may produce as marked an effect on the gradient as changes in 
intensity, for they bring the observer into another part of the 
area. Many illustrations of change in gradient will be found in 
Plate I, which is the tracing of a self-registering barometer for 
the month of May, 1893. 
The characteristics of the cyclone which should be remem- 
bered in this connection are wind easterly in front, westerly in 
the rear; gradually thickening cirrus clouds in front, with cumulus 
