x 
36 M. Dove on the theory of the Variations 
6. In middle and western Europe the barometric pressure 
appears to decrease everywhere from the month of January to 
the spring, usually attaining a minimum in April; it then rises 
slowly but steadily to September, = sinks rapidly to November, 
_when it usually reaches a second minimum. In summer, there- 
fore, the whole atmospheric pvionire gains more by increased 
evaporation than it loses by expansion. . This over-com pensation 
is probably to be explained, as we have seen above, by the lat- 
eral overflow received in the upper regions from Asia. In Sitka 
the whole annual curve is convex, a result only found in Europe 
. considerable mountain elevations, where it is a consequence 
the expansion, and extension sia Hal of the whole mass of 
oe atmosphere in summer. 
The region of great annual barometric variation, on the: 
vs 
Asiatic side of the globe where monsoons prevail, ex tends much 
further to the north in the northern hemisphere, than it does to 
the south in the southern hemisphere; for the variation reaches . 
its maximum at Pekin, while at Hobarton, in nearly a correspond- 
ing latitude, it has already become inconsiderable; and it is gen- 
erally greater in the northern than in the corresponding southern 
latitudes. The exact contrary is the case on the Atlantic side 
and in the region of the Trades ; for here the annual variation, 
though nowhere very considerable, is decidedly greater in the 
southern than in the northern hemisphere, as is shown by the re- 
sults of observation at the Cape, Ascension, St. Helena, Rio Ja- 
neiro, and Pernambuco, compared with the West Indian Islands 
and the southern parts of the United States. Hence it follows, 
that if we compare places in the same latitude, we find but little 
difference between the annual variation in the southern Atlantic 
and southern Indian oceans, while in the northern hemisphere we 
have in the same latitude the very large annual variation in the 
north part of the Indian and in the Chinese seas, and the almost 
entire absence of aunnal variation in the Atlantic (compare Chu- 
san with the Azores and Madeira). The explanation of the last 
named phenomenon, 7. e. that of the northern hemisphere, by a 
lateral overflow in the upper parts of the atmosphere, seems so 
direct, that I think we may pronounce the irregular form of the 
annual barometric curve in the West Indies to be a secondary 
phenomenon, the primary causes of which must be looked for on 
st. 
8. It is known that in the eruption of the Coseguina on the 
20th of January, 1835, when the isthmus of Central America 
was en by an earthquake, not only were volcanic ashes car- 
ried to Kingston in Jamaica, a distance of 800 English miles 
in the opposite en to the trade wind, but some of the 
sam me ashes also fe 700 miles to the westward, on board the 
Conway, in the Paci Ocean. We infer, therefore, that in 
OE Eps STE ey SIG SEES 2 ie at a aad Eee NNR Simeas ag eNO iar hia iat 
