Ground in changing Currents of Wind. 97 
trees is retarded by the friction of the air on the leaves and 
branches. When wind has to rise over a ridge of ‘ground, 
there is the friction of the air on the earth’s surface, and if 
all the air which ascended the ridge were to descend on the 
other side to the same level, so that the opposite surfaces of 
ridge resembled the two arms of a syphon, then the friction 
would be the only retarding force. But this is not what oc- 
curs in nature, which will be at once seen if we look ata 
mountain ridge on a great scale. Take for illustration the 
island of Great Britain ; a large mass of air annually passes 
across its surface from the Atlantic to the German Ocean ; 
in its transit a quantity of aqueous vapour is condensed, and 
descends in the form of rain. A part of the rain is derived 
from vapour which has been evaporated from the surface of 
the island, and is only a process of return—the state of the 
earth as to moisture, in an annual mean, being nearly sta- 
tionary. The extent of the condensation of vapour which has 
come from the sea or foreign ground is measured by the an- 
nual discharge of our rivers. The vast volume of air annu- 
ally condensed on the surface of the island may be conceived, 
when the quantity of water discharged into the sea is multi- 
plied 1700 times ; and the winds have suffered from the dis- 
charge of their aqueous vapour a retarding force measured 
by the gravitating of the rivers in their descent to the ocean, 
for the weight of water is lost to them on the descending 
limb of the syphon. 
If -we reflect on the extent of the water-power of this 
island, some idea may be formed of the vast body of air re- 
quired to be condensed to produce it. Again, this formation 
of water-power brings another force into’ play which tends 
to give a horizontal motion to the air, namely the vacuity 
left by the condensed moisture. Consequently when we 
come to consider the effect of elevated ground on the atmo- 
sphere in its widest sense, experience shows that it acts as a 
disturber ; the most palpable proof of this is afforded by the 
general quiescent state of the extensive flat surface of the 
Pacific Ocean,—the name pacific being an indication of its 
haracter—contrasted with the stormy portions of this ocean 
hich lie around Cape Horn and the Cape of Good Hope. 
VOL. LVII. NO. CXIII.—JULY 1854. G 
