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work or ‘‘ MAUD’? EXPEDITION—SVERDRUP 225 
three curves, (1) representing the velocities over the North Atlantic 
trade-wind region, (2) over middle Europe, and (3) over the part 
of the Arctic which we have traversed. I wish to draw your atten- 
tion to the wind maximum which the last two curves show at great 
“MAUD” DURING DRIFT OCTOBER 1922 TO APRIL 1523 : 
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"MAUD" DURING DRIFT OCTOBER 1923 TO MAY 1924 
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SCALE OF GREENWICH MEAN TIME IN HOURS 
DIURNAL VARIATION OF | THE POTENTIAL GRADIENT OF THE ATMOSPHERE © 
Fig. 2 
altitudes. ‘This maximum is known to occur at an important bound- 
ary surface of the atmosphere, which has been called the “ceiling 
of the troposphere.” Below the maximum, within the region called 
the troposphere, the temperature decreases with altitude, but above 
