DEVELOPMENT OF METEOROLOGY—ABBE. 307 
finds the average temperature of the air at sea level] at the place of the 
spout was 67.5° F., or the maximum for the day, and the therme- 
graph for Nantucket also shows that the spout occurred at the time 
of maximum temperature. On the other hand, the temperature at 
the land stations fell rather rapidly to 56.5° at Vineyard Haven and 
59.0° at Woods Hole, so that the effective temperature within the © 
anticyclonic wind that prevailed around the outside of the cloud, 
or at a distance from the spout, was about 58°. The barometric 
pressure in this outside region was about 30.10 inches, but it must have 
been about 30.05 near the waterspout. The relative humidity was 
low at the meteorological station. The lower strata of the atmos- 
phere were drier than on any other day of the month, and after 
several trial computations Professor Bigelow accepts a relative 
humidity of 64 per cent as prevailing in general near the surface of 
the water at the time the waterspout was formed. These are the 
meteorological data at sea level beneath the cloud which surmounted 
the waterspout. This cloud was a large cumulo-nimbus, with its 
flat base about 3,600 feet above sea level, as just stated in connection 
with the length of the tube. With these meteorological data and the 
thermodynamic equations, Professor Bigelow computes the condi- 
tions in the air ascending in a rapid whirl within the center of the 
tube. 
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 up- 
ward for 5,669 feet, or to a total height of 9,206 feet. Here the 
freezing 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 
erystals, 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 verti- 
cal gradient corresponded 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 
