5 02 THE POPULAR SCIENCE MONTHLY. 



When wind blows over water, all the air does not pass over the 

 surface of the water. On account of the high degree of adhesion 

 between air and water, a thin stratum of air remains in contact 

 with the water, and it is the action of the internal friction or vis- 

 cosity of air tending to draw this stratum along which causes the 

 tractive effect of wind on water. 



When a film of oil is spread over the surface, this tractive 

 force is not brought to bear on the surface of the water as long as 

 the film remains unbroken, but acts upon the surface of the film, 

 whose particles, being entirely separate from the particles of 

 water, do not share their motion. The surface of the water is 

 thus shielded from the action of the wind in the same manner as 

 if a skin of India rubber were spread over it, and the only action 

 of the wind in such a case is to move the film over the surface of 

 the water. 



It is calculated that a wind moving at the rate of twenty-five 

 miles per hour or one hundred and twelve centimetres per sec- 

 ond, relatively to the surface of the water, exercises a tractive 

 effect of about two thousandths of a gramme upon each square 

 centimetre of surface ; and, when we consider that this force is 

 brought to bear upon a system of particles moving in their orbits, 

 in the direction in which the wind blows, with a speed of about 

 eighty centimetres per second, it will be apparent that the inter- 

 position of a film of oil between the air and water must have a 

 powerful effect in preventing breaking crests. 



Observation has shown that, in the generation of oscillatory 

 waves, ripples or capillary waves are first formed, and that it is 

 to the union of conterminal ripples and to their more abundant 

 formation with the increased force of the wind that the growth of 

 waves is due. The existence of a certain definite tension, equal to 

 '08235 gramme per lineal centimetre, at the common surface of air 

 and water has been pointed out. The water surface under this 

 tension is in perfect equilibrium. 



When wind blows over the surface of a body of water, the tan- 

 gential force which the air, in virtue of its viscosity, exerts on the 

 surface of the water, is of different degrees of intensity at different 

 places, owing to the minute corrugations which are always pres- 

 ent on the surface of a body of water, and to the eddying motion 

 of the air. At the places where the tangential force is greatest, 

 the surface film of water is drawn along and the portions of the 

 surface immediately in front of them, destroying their surface 

 tension or energy of position, and, by laying bare new surface in 

 places from which they are moved, generating a like amount of 

 surface tension. Through this action heaps or ripples are formed, 

 and surface tension is being constantly generated and destroyed. 

 The formation of ripples takes place on waves already in exist- 



