METEOROLOGY. 193 
the sense just referred to. It simply means the limit of perpetual snow; 
while the lower snow line refers to a limit at which snow sometimes 
continues for some years. 
But little change in the temperature of springs is observed with the 
different seasons, the range in the temperate zones being rarely more than 
from 1°—3°F. Their highest temperature in the northern hemisphere 
occurs in September, their lowest in March. In the torrid zone their mean 
temperature is somewhat lower, in the temperate somewhat higher than that 
of the air. Springs coming from great depths have. however, a much 
higher temperature, as shown by various salt and mineral springs, and 
artesian wells. 
As already observed, the daily variations of temperature are much slighter 
at sea than on land. In the torrid zone, the difference between the daily 
maximum and minimum, at the highest, amounts to 2°—4°F. ; in the tempe- 
rate zone to 3°—6°F. In the torrid zone, the temperature of the sea 
decreases with the depth; while at the surface this may amount to from 
80°—88°F’., at great depths it may be only 35° or 36°F. This coldness of the 
lower strata of the ocean waters, according to Humboldt, causes currents 
which lead the polar waters towards the equator. 
> 
3. Atmospheric Pressure. 
The amount of pressure exerted by the atmosphere at any one place is 
measured by the height of the mercury in the barometer. Temperature, 
however, also affects this height, the mercury rising slightly with increase 
of heat, and sinking with cold, independently of any change in the pressure 
of the air. To compare different observations, therefore, it becomes 
necessary to reduce them to some standard of temperature, this being 
_ assumed at 32°F, or the freezing point. Now mercury expands .0001 of its 
bulk for every degree above 32°F., therefore all that is necessary to make 
the required reduction is to subtract the ten-thousandth part of the observed 
altitude for every degree of Fahrenheit above 32°. Application may also be 
made of the formula h— (¢—32) X 4+ .0001 =the corrected height, 
where h = the observed height of the mercurial column, and ¢ = the tempe- 
rature at the time of observation. Allowance must also be made for the 
expansion of the attached scale, unless this be of glass, brass, or other 
metals expanding nearly as much as mercury. A third correction must be 
made for capillary depression of the mercury. For the sake of avoiding 
tedious calculations, tables of these corrections required have been 
constructed. 
The barometer exhibits incessant variations in altitude, distinguished into 
diurnal and annual. Of these the former are much less conspicuous in the 
temperate zones than in the torrid. There the amplitude or greatest extent: 
of daily variation is, in Quito, 2.82, in Guinea 2.44, in Cumana over 23, in 
Jamaica 1.45, m the Canary Islands 1.10, in Rome 0.70, in Paris 0.76, in 
London 0.38, and in St. Petersburg only one fifth of a millimetre. In 
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