17i PNEUMATICS. 



other ten degrees being occupied by wind from the 

 opposite point of the compass ; or N. W. ; at the 

 same time that the winds in all the northern parts 

 of the Indian Ocean shift round, and blow di- 

 rectly contrary to the course they held in the 

 former six months. These winds are called mon- 

 soons, or shifting trade-winds. 



These changes are not suddenly made. Some 

 days before and after the change, there are calms, 

 variable winds, and dreadful storms, attended with 

 thunder, lightning, and rain. 



On the greater part of the coasts of lands 

 situated between the tropics, the wind blows to- 

 wards the shore in the day-time, and towards the 

 sea in the night. These periodical winds are 

 termed the land and sea breezes. 



When several winds converge swiftly to one 

 point, the air ascends with greater rapidity, and 

 acquires a whirling motion, like that of water de- 

 scending in a funnel. And as the centrifugal 

 force in this whirling motion of the water, is often 

 sufficient to counterpoise the lateral pressure, and 

 to prevent its approaching the central part, it 

 frequently happens, that a perforation is seen quite 

 through the body of the fluid. In like manner, 

 the centrifugal force of the air may become equal 

 to the pressure of the atmosphere, and conse- 

 quently leave a void space about the centre of the 

 motion. The phenomenon is called a whirlwind, 

 and sometimes produces fatal effects. For, partly 

 by the expansion of the air included in houses and 

 other buildings; and partly by the violence of the 

 ascending current, it happens that bodies near the 

 centre of the whirl are blown up into the vacuum, 

 or carried aloft with great impetuosity iu a spiral 

 motion. 



