METEOROLOGY. 189 
these currents, small balloons, like those used so successfully by Thomas 
Forster, are very useful ; in most cases four to five, and sometimes even 
seven to eight simultaneous different currents may be found to exist. 
According to the observations of Placidus Heinrich, instituted at 
Regensburg (Ratisbon) in the months of May, June, and July, 1791, the 
North wind blew 9 times above, 11 times below 
RUMEN BMRORSAR Hit iii. oe sa ng apie 
ae aire) HRY TRAE N98) ice dQ 9EOh 
SHE wires eames AQ “ah Oe 
HT Mempn inde: Ines aly YBth “Lowa 
SAE Siene walt Sout, sly cle ab Dae Hips 
A) TRIER Sr oo OO aOR trie 
INE eee! 1 aT ti, Mond deantrans vais 
The trade wind, which in the torrid zone blows constantly from east to 
west, may be readily understood from what has already been stated. In 
the vicinity of the equator, the air being greatly heated, expands and flows 
above towards the poles, at the same time that a counter current of cold 
air sets in from the poles to the equator. Had the earth no rotation on an 
axis, there would be a north wind on the northern hemisphere, and a south 
on the southern. But the earth turns on her axis from west to east once 
in twenty-four hours, and individual regions have an actual velocity which 
is greater with the vicinity to the equator. The air coming from the poles, 
and preserving its original velocity, must move slower towards the east 
than the earth itself, and consequently will have, in respect to the latter, a 
relative motion from east to west. This motion, combined with the 
motion from the poles to the equator, gives as its resultant a north-east wind 
to countries north of the equator, and a south-east wind to those south of 
it. These winds blow over the whole earth, but are generally only sensible 
at sea, at some distance (about fifty miles) from the main land. In the 
Atlantic the trade wind extends from 8° to 28° or 30°; in the Pacific to 
25° N.L.; the limits im the southern hemisphere are less accurately 
determined. Sailors make use of these winds in the voyage from Europe 
to America, by steering south from Madeira nearly to the tropic of Cancer, 
whence, with little effort of the crew, they are carried west. In like 
manner the Spanish vessels went regularly from Acapulco to Manilla with 
this wind. Kaemtz assumes the limits of the north-east trade wind at 
2° and 23° N. (the latter moves in summer a little more northerly, and in 
winter a little more southerly), that of the south-east trade at 2° to 4° and 
21°S., the northern limit of the latter lying in the Atlantic Ocean in the 
northern hemisphere. In the vicinity of the equator the two trades become 
more easterly, and are separated by a zone in which there is generally an 
almost total absence of wind, hence called the region of calms. This has a 
mean breadth of about 6° (in August of 92°, and in December only of 24°), 
and lies somewhat north of the equator, from which its northern limit is a 
little further removed in summer than in winter. Nevertheless, the calms 
alternate incessantly with sudden squalls or storms and irregular winds. 
In the Indian Ocean the regularity of the trade wind is disturbed by the 
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