78 SCIENCE OF COMMON THINGS. 



Movements of clouds. Trade winds and their location. 



488 Why do we sometimes see clouds at one elevation moving in one 

 direction, and at anotfier elevation, at the same time, others moving in a 

 contrary direction ? 



Because different currents of air exist at different 

 elevations, moving in different airections, with different 

 velocities. 



In 1839, an English aeronaut, at the height of 14,000 feet, encountered 

 a current that bore him along at the rate of five miles per hour ; but, 

 upon descending to the altitude of 12,000 feet, he met with a contrary- 

 wind, blowing with a velocity of eighty miles per hour. 



489 How is the force of the wind ascertained ? 



By observing the amount of pressure that it exerts 

 upon a given plane surface perpendicular to its own 

 direction. 



If the pressure plate acts freely upon spiral springs, the power of the 

 wind is denoted by the extent of their compression, and that weight will 

 be a measure of their force, the same as in weighing by the ordinary 

 spring-balance. 



09 O What is an instrument for measuring the force of the wind called f 



An Anemometer. 



491 What are the constant winds which bloiv over the Atlantic and 

 Pacific Oceans called f 



They are called " trade-winds" 



493 Why are they called trade-winds f 



Because they are very convenient to navigators who 

 have to cross the ocean, inasmuch as they always blow 

 in one direction. 



493 In what direction do the trade-winds blow f 



That in the northern hemisphere blows from the 

 north-east ; that in the southern hemisphere from the 

 south-east. 



494 Do trade-winds blow from the north-east and south-east all Hie 

 year round 1 



Yes, in the open sea; that is in the Atlantic and 

 Pacific oceans, for about 25 each side of the equator. 



495 Where do the trade-winds blow with uniform force and constancy f 



In many parts of the Pacific embraced within the 

 region of the trade-winds, a vessel may sail for a week 

 without altering the position of a sail or rope. 



