52 Meteorological Sketches. 
which, with some unimportant exceptions, appear to prevail through- 
out the globe. These movements evidently show the continued 
operation of some powerful impulse, which, to the writer at least, 
does not appear to have been satisfactorily explained. It is esti- 
mated from the average rate of sailing of ships during long voyages 
through different seas, and from other data, that the average velocity 
of the wind near the surface of the ocean is equal to eighteen miles 
an hour throughout the year, and in the common region of the clouds 
the velocity must be much greater. 
Temperature of Elevation. 
Elevation above the level of the sea, or the general level of a 
country, causes a regular variation in temperature. The first 300 feet 
usually cause a difference of about 1 degree of Fahrenheit’s ther- 
mometer. After ascending 300 feet, it is estimated that the thermom- 
eter falls a degree in 295 feet, then at 277, 252, 228, and 192 feet ; 
but 300 feet to a degree isa common rule. On these principles the 
limit of perpetual frost has been calculated. It is made a little more 
than 15,000 feet at the equator, and from that to 13,000 between 
the tropics, and from 9,000 to 4,000 feet between latitudes 40° 
and 59°. 
It has been found, however; that the above rule is subject to great 
variations, owing, probably, to the course, temperature, and super- 
position of the atmospheric. currents which prevail in different re- 
gions, and at different altitudes. Colder currents are often found 
resting upon, or interposed between, those of a higher temperature, 
and vice versa. On the Himalaya Mountains, in Asia, between the 
latitudes of 28° and 34° north, the region of vegetation has been 
found to extend several thousand feet above the supposed line of 
congelation in those latitudes. It is also remarkable that the line of 
perpetual snow is found at a much greater altitude on the northern 
side of these mountains than on the southern side in a lower lati- 
tude. From this it may be inferred that the temperature in high 
regions, as well as in lower situations, is greatly affected by the geo- 
graphical course and physical condition of the currents of atmos- 
phere which prevail in those regions. 
Stratification and Elevation of the Currents of the Atmosphere. 
It is obvious, from the courses of the clouds and other light bo- 
dies which sometimes float in the atmosphere, that the movements 
