EF. Loomis— Contributions to Meteorology. 259 
important conclusions. If it should be asked what is the cause 
of areas of high pressure, the answer is plain, they are the nec- 
essary concomitants of areas of low pressure. If at any place of 
the earth’s surface the barometer sinks below its mean height, 
it must rise above its mean height at some other place. If we 
find an area of low pressure of vast extent, we know that there 
must be a corresponding area of high pressure- in some other 
part of the world. For the entire globe, the total amount of air 
which at any instant is elevated above the mean level surface 
must be exactly equal to the. total amount which in other 
regions is abstracted from beneath the mean level surface. 
Areas of high pressure are therefore built up by air which 
comes from areas of low pressure. We also know that within 
an area of low pressure at the surface of the earth, there is a 
general movement of the air anward ; that near the central por- 
tion of this area the air ascends and flows off to other portions 
of the globe. Also within an area of high pressure at the sur- 
face of the earth, there is a general movement of the air outward ; 
and since high areas often continue many days and sometimes 
many weeks, there must bea fresh accession of air at the top to 
supply the place of the air which is drawn off by the outward 
movement near the surface of the earth. The air which ascends 
from an area of low pressure goes therefore to maintain the 
descending movement within av area of high pressure. This 
deduction seems so obvious as scarcely to require confirmation 
from direct observations. It has however been abundantly 
confirmed by an extensive series of observations on the move- 
ment of cirrus clouds made in 1875 and ’76 under the direction 
of Dr. Hildebrandsson. By these observations it has been 
shown that the air which ascends from an area of low pressure, 
aiter reaching an elevation corresponding to the usual height of 
the cirrus clouds, recedes from the center of the low area, and 
moves toward an area of high pressure. 
When we attempt to apply this general principle to a particu- 
lar case we are frequently in doubt as to the particular low area 
from which a given high area draws its supply of air. In the 
observations collected by Dr. Hildebrandsson, when a low area 
prevailed over the Atlantic Ocean and a high area over Hastern 
Kurope, the cirrus clouds generally moved toward the area of 
high pressure on the east side; but when a low area prevailed 
over Hastern EKurope with a high area over the Atlantic Ocean 
the cirrus clouds generally moved toward the area of high 
pressure on the west side. It is noticeable that the period of 
greatest barometric pressure over Huropasia corresponds with 
the period of least pressure over the Atlantic Ocean. There 
does not therefore seem to be any room for doubt that the high 
areas of Huropasia derive their chief supply of air from the low 
areas which prevail over the Atlantic Ocean. 
