HYDROGRAPHICAL SUHVEY. — WAVES. 221 



and extent of the sea in which they take their rise, 

 and on tlie state of dryness and degree of pressure 

 of the atmosphere. 



The natural progressive motion of waves being 

 in the same direction as the wind, the windward or 

 " weather-side" of a lake, river or sea, is generally 

 almost as smooth in a storm as in a calm. Waves 

 increase in size, accordingly as the strength of the 

 wind and the distance from the windward-shore, 

 become greater ; but after they attain a certain 

 magnitude, any greater distance from shore is pro- 

 ductive of no further increase in size. Then they 

 move forward, maintaining a similar elevation and 

 velocity, (excepting where they are accidentally 

 augmented by two or three waves of different ele- 

 vations overtaking one another, and combining in 

 tlie formation of one great sea.) to a distance often 

 of many leagues beyond the limit to which the wind 

 that produced them extends. 



Were the atmosphere "without pressure, it is pro- 

 bable, that no waves, unless of the smallest kind, 

 would-be produced. The pressure of the atmos- 

 phere bringing the wind and sea into immediate 

 contact, produces more or less friction in proportion 

 to the dryness or dampness of the air. Thus, when 

 the air is very dry, it possesses a great attraction 

 for water, passes over the surface of the sea with 

 more friction, and produces more considerable waves; 

 but whenever it is saturated with moisture, the at- 



