AND OTHER WATER WAVES 359 



for the trees " i.e., one cannot see the group for 

 the waves. At a distance, when the trees are no 

 longer individually discernible, one makes out the 

 size and shape of the wood. Similarly, when one 

 can no longer distinguish individual curved waves 

 one sees at once in startling distinctness Lord 

 Kelvin's straight -lined boundary, marking the limit 

 to which the waves appreciably extend. In 

 addition to this group -line there is another, also 

 originating at the ship, which marks the rear of the 

 diverging series of waves. Thus the wave-track 

 of the ship is generally reduced at a distance to 

 four straight lines, radiating from the ship, two 

 on either side. 



When this is the appearance presented to the 

 naked eye a pair of field-glasses reveals the 

 individual diverging waves. Sometimes also, 

 especially when the ship is directly receding, the 

 space between the inner boundaries of the diverging 

 waves is seen to be filled up by a cross -biarring 

 of the flatter and broader thwart waves. 



The visibility of the waves depends upon cir- 

 cumstances of reflection, and these vary much with 

 the direction of the ship's course relatively to the 

 observer and to the bright and dark parts of the 

 sky and land. Thus, it sometimes occurs that the 

 straight line forming the outer boundary of the 



