308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
hundred feet. This small difference of 0.003 inch per hundred feet 
corresponds to a total difference of 0.11 inch between sea level and 
the cloud base 3,600 feet above. It is this difference of pressure that 
is the effective gradient for vertical movement, causing the air at 
the outer boundaries to slowly descend while the air within the tube 
‘apidly ascends. 
The main part of Professor Bigelow’s memoir is devoted to ex- 
plaining numerically each step in the formation of the spout and 
its linear and vertical motions over Vineyard Sound. From this 
special study he is led to investigate the whole question of the con- 
dition attending any overturning that may occur in the atmosphere. 
If a layer of cold air be spread over a layer of warm air, resting 
quietly upon it with the help of an intervening diaphragm, and the 
latter be removed, we all know that the cold air must descend and 
the warm air rise—a process of overturning such as is occurring 
every day in the atmosphere. The mechanical conditions or mechani- 
cal theory of this upsetting were recently worked out by Margules, 
and his views, with some important modifications, are developed by 
Professor Bigelow in such a way that a certain conclusion is inevi- 
tably reached. This overturning takes place not merely in a small 
way, as in thunderstorms, but on the grandest scale in tropical hur- 
ricanes. Now the question has been discussed pro and con for a 
hundred years as to whence comes the energy involved in the pro- 
duction of the rapid rotary winds of hurricanes. Espy maintained 
that in thunderstorms this energy was derived from gradients due 
to the condensation of aqueous vapor and the evolution of heat in 
the clouds. I thought it due also largely to the sun’s heat acting 
on the top of the cloud. Professor Bigelow shows that while these 
are true causes, yet for hurricanes they are entirely insufficient, and 
that the energy of these great storms is mainly derived from the 
gradients produced by the overturning of layers of cold air flowing 
from northern latitudes over the warm air that is flowing from 
southern latitudes: by the descent of this cold air to the ground the 
force of gravity gives it great velocity and momentum. In other 
words, we must not look upon a great storm as a symmetric cyclone 
with a center of warm rising air and an inflowing pericyclone of 
cold air, as was taught by former meteorologists, but we must face 
the problem of a simple overturning in the lower strata of the 
atmosphere below the level of the general west wind that is flowing 
a few miles above us. The ideal cyclone and anticyclone probably 
do not exist in the atmosphere. This conclusion gives precision to 
an idea that Ferrel fully acquiesced in, namely, that the atmosphere 
has no simple circulation, cyclonic or anticyclonic, but is a complex 
mass of interlacings of currents; so that the progress made by him- 
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