54 
ABBE. 
The preceding dimensions, computed trigonometrically, 
have been quoted as measured from the photographs, but the 
figures deduced from thermodynamic theory and Professor 
Bigelow’s tables are as follows: The height of the base of 
the cloud, or the dry stage of the ascending air, should be 
3,537 feet, or 63 feet less than the 3,600 measured on the 
photograph. The cloud stage extends thence upwards for 
5,669 feet, or to a total height of 9,206 feet. Here the freez- 
ing or hail stage begins, which is a comparatively thin layer 
of only 243 feet, and therefore ceases at a total height of 
9,449 feet. Above this all precipitation is in the shape of 
snow, or minute crystals, certainly not hail or frozen water- 
drops, and the thickness of this layer, 6,765 feet, brings us 
to the top of the cloud, at 16,214 feet, or about 5,000 meters 
above sea-level. The agreement of these thermodynamic 
computations with trigonometrical measurements is quite 
satisfactory. 
Now the motion of the air depends essentially upon the 
change of pressure, or the gradient. An abnormal horizontal 
gradient will produce horizontal motion or whirlings, but a 
vertical gradient will produce rising or falling motion of the 
air. Only a short distance from the waterspout, over the 
island of Nantucket proper, the vertical gradient corre- 
sponded to a fall of 0.098 inch for each ascent of a hundred 
feet, whereas the temperature and moisture conditions over 
the water near the spout give a vertical gradient of 0.101 
inch per 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 bounda- 
ries to slowly descend while the air within the tube rapidly 
ascends. 
The main part of Professor Bigelow’s memoir is devoted to 
explaining 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 condition attending any overturning 
that may occur in the atmosphere. If a layer of cold air be 
