TRANSACTIONS OF SECTION G. 



773 



their -works, and so see if tliey are built in accordance -witli some definite rule or 

 formula, or if they have learned by long experience what is safe for each type 

 and each material. 



1. On Recording Sotmdings by Photography. By J. Dillon. 



5. On the Size of Waves as observed at Sea. 

 By Vaughan Cornish, D.Sc. 



The Height of Waves. — The height of the ocean -waves in deep -water from 

 land has been determined with faii-ly concordant results by independent observers. 

 The values recorded are the average of the heights of a number of successive 

 waves : — 



These values are only about one-half of the 40 or 50 feet -which experienced 

 seamen frequently state to be ' the size of the waves ' met with in strong gales in 

 the open ocean. The author has observed during gales in the North Atlantic 

 that waves of a larger size recurred at short intervals, and that it was these 

 which riveted the attention and which were dangerous. He thinks that it is the 

 average size of these ' ordmary maximum ' waves which is commonly estimated 

 by seamen as 40 to 50 feet, and he suggests that it is desirable to record in 

 future, not only the general average height, but also the height of the ' ordinary 

 maximum' waves. This practice would do away with much of the apparent 

 discrepancy between the accounts of the size of waves at sea, and would also give 

 some notion of the simultaneous diii'erences of roughness at different points, which 

 is an important aspect of a sea-way. 



The Length of Waves. — The highest waves in deep water are recorded during 

 otorms, but the longest are the swells encountered in a calmer atmosphere. At 

 sea, where the ship rises and falls, and there is no fixed object to provide a datum 

 line, crests and troughs are judged less by actual elevation than by convexity or 

 concavity of the water's surface. "When the profiles of two waves of nearly equal 

 amplitude but of very diflferent wave-length are combined, the resulting wavy 

 line presents a series of inequalities the wave-length of which is fairly regular, 

 and equal, on an average, to that of the shorter component. When, however, the 

 two combining waves, of very ditierent wave-length, are of equal stee^mess, the 

 combination appears as a series of inequalities which, although displaying minor 

 sinuosities of outline, have unmistakably the wave-length of the longer com- 

 ponent. Their average amplitude is also equal to that of the longer component. 

 This indicates (a) that a swell, even of great amplitude, is not directly measure- 

 able in a storm ; (b) that a great swell scarcely affects the recorded average height 

 and length of the shorter storm-waves, but that it can cause irregularity of the 

 kind referred to in the last section ; and (c) that the appearance of the water may 

 change somewhat suddenly from that of an irregular short sea to that of an 

 irregular long swell, the longer component being now what the author terms 'the 

 dominant wave.' This change of appearance is not, however, accompanied by any 

 acceleration of the processes going on in the wave-water. 



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