152 ELEMENTS OF SCIENCE 



shores becoming cooler than the sea, we have an opposite 

 effect, a breeze from land to sea. Hill and valley breezes 

 also exist, partly due to mountain summits receiving more 

 heat by day, and radiating it far more readily by night 

 than do the lowlands, arid partly to colder and therefore 

 heavier air rolling down hill. 



Rotatory storms, hurricanes, and typhoons may have a 

 diameter of four or five hundred miles. Such move- 

 ments are not so many transfers of the mass of air itself, 

 but are rotatory movements of great velocity trans- 

 mitted through its particles. They are supposed to be 

 occasioned by a rapid movement of ascent communicated 

 to air by some very heated spot of the earth's surface, such 

 movement taking place through air either relatively at 

 rest or moving in a contrary direction. This would be 

 sufficient to occasion an incipient vortex which may be 

 compared with the conical vertices before considered* as 

 occurring in water. Rotatory storms wander about with 

 a movement of translation which is slow when compared 

 with the enormous rapidity of rotation, which may be 

 more than ninety miles an hour. These aerial vortices 

 proceed obliquely north and south from the region of the 

 equator, turning from west to east, while their move- 

 ment of translation, in the northern hemisphere, is west- 

 ward till they have passed the region of the trade winds, 

 then they turn eastwards. 



There is an instrument for measuring the wind's force 

 (anemometer or anemoscope) consisting of an upright 

 U-shaped tube, containing a little water, with one end 

 bent horizontally so as to face the wind. A scale for 

 registering the height of the water is placed between 

 the two limbs of the upright tube, the water being, of 



* See ante, p. 74. 



