482 POPULAR LECTURES AND ADDRESSES. 



exceedingly small imperceptible indeed until 

 you come to these two definite lines. You see the 

 oblique wave-pattern waves in echelon pattern. 

 The law of that echelon is illustrated by the curves 

 shown in Fig. 48. The algebraical equations of 

 these curves are 



'-V 



FIG. 48. Echelon curves. 



where x and j', according to ordinary usage, are 

 measured along, and perpendicular to, the direction 

 of motion from E towards A, and w is an arbitrary 

 variable ; by assuming a series of arbitrary values 

 for w, a corresponding series of values for x are 

 found from the first equation, and thence the 

 corresponding values of y from the second. I 



