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198 PHYSICS. 
uncertainty. This consists of two parallel glass tubes about eight or nine 
inches long, half an inch in diameter, and connected beneath by a bent 
tube of one tenth of an inch bore. A metal cap, bent at right angles, is 
attached to the upper end of one tube, its opening receiving the wind. 
Both legs are so fastened to a vertical iron rod as that the wind keeps the 
whole instrument in the direction of its current. Water is poured into the 
tubes, filling them about half full. When the wind blows into the mouth 
of the metal cap, it depresses the water in one tube, causing it to rise in the 
other. The difference of level between the two tubes gives the height of a 
column of water whose weight is equal to the force of the wind acting on 
a surface of equal area. This anemometer gives approximately very 
accurate results. ‘There are, however, three difficulties in the way of its 
general use: unequal temperature, evaporation, and the freezing of the water ; 
on this latter account it were better to make use of dilute sulphuric acid. 
According to the determinations of Rouse, the velocity of an almost 
insensible wind amounts to one and a half feet in a second ; of one just 
perceptible, three to four and a half feet ; of a pleasant wind, six to seven 
and a half; of a brisk wind, fourteen and a half to twenty-two; of a very 
brisk wind, twenty-nine and a half to thirty-six and a half; of a strong 
wind, forty-four to fifty-one and a half; of a very strong wind, fifty-eight 
and a half to sixty-six; of a storm, seventy-three and a half; of a violent 
storm, eighty-eight ; of a hurricane, 117; and of the most violent hurricane, 
to 147 feet per second. Brandes estimated the velocity of a moderate wind 
at twelve to sixteen feet; Woltmann, that of a violent storm at seventy to 
eighty feet; Rochan, that of a tropical storm at 150 feet; the maximum 
being 600 feet. Kraft determined the velocity of the wind during a violent 
storm in St. Petersburg to be 110 to 123 feet. Finally, the wind appears 
to increase in velocity with the elevation. 
Kaemtz has divided the winds into regular and irregular or variable. 
To the first class belong the land and sea breezes, the trade winds, and the 
monsoons. and and sea breezes are experienced on coasts and islands, 
and are produced by causes already explained. After sunrise the land is 
heated to a greater degree than the sea; consequently, about ten A.M. a 
current sets in from the sea—a sea breeze—which reaches its maximum of 
intensity between two and three P.M., a calm again ensuing about sunset. 
Equilibrium of temperature between the two is soon disturbed by the 
greater depression of temperature on land produced by more rapid 
radiation ; this being now cooler than the sea, a current of colder air pours 
out seawards from the land—the land breeze—which lasts until about eight 
A.M. In both cases an opposite current exists in the higher regions of the 
atmosphere. The land breeze is inconsiderable about peninsulas, but very 
sensible in bays. Similar periodical variations prevail also ou the banks 
of great lakes, as also in long valleys and ravines. 
It very frequently happens that currents of air, at different heights, move 
in entirely different directions, as shown by the motion of clouds of different 
degrees of elevation. This is illustrated by pl. 24, jig. 12, where ab is the 
direction of the lower, dc that of the upper current. In investigations into 
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