The Ocean as an Acoustic System 



113 



also that the position of Cold Fronts on weath- 

 er maps is only a rough geographical location 

 — and good only within a period of a few hours. 

 It is therefore impossible to state within min- 

 utes just when a cold front first enters the 

 Atlantic. That pronounced microseismic ac- 

 tivity is recorded when a cold front enters the 

 Atlantic we most heartily agree. One state- 

 ment of yesterday therefore in no way contra- 

 dicts those of Dr. Press. We do state, how- 

 ever, that a matter of hours before this (the 

 precise number of hours depending on how 

 fast the cold front is advancing) we record 

 microseismic activity caused by the front as it 

 passes over the Great Lakes. We record this 

 in a matter of minutes after the front has 

 reached the Lakes — giving the seismograph a 

 definite warning value in the case of fronts. 



In a sentence, the contradiction is ex- 

 plained by pointing out that we are recording 

 waves from the frontal activity over the Lakes, 

 whereas Press and Donn are referring to waves 

 from the frontal activity over the Atlantic. I 

 merely wish to emphasize that we are all in 

 agreement on the microseismic activity as the 

 front passes over the Atlantic. 



Longuet-Higgins. (1) The response curve 

 for the movement of the ground shown in 

 Figure 1 is considerably sharper than that 

 shown in my paper (Figure 7). The reason is 

 probably as follows: the first curve is the re- 

 sponse to a horizontal plane oscillation of in- 

 finite extent, which causes energy to be propa- 

 gated vertically downwards into the ground ; 

 the other is the response to a pressure distribu- 

 tion of finite extent from which the waves are 

 propagated outwards horizontally. The first 

 waves are relatively difficult to generate, being 

 subject to less constraint. 



(2) Dr. Press has pointed out the rather 

 sudden onset of microseisms at the time that a 

 cold front crosses the coast. I think that there 

 is no difficulty at present in supposing that 

 this is due to wave interference. As the weath- 

 er maps show, there is then a very sharp 

 change in the direction of the wind. It is not 

 necessary that the wind should be exactly re- 

 versed in direction, because a given wind will 

 probably generate waves which, when analyzed, 

 will be found to have some wave components 

 travelling at a considerable angle to the mean 

 direction. N. F. Barber has shown by an 

 optical diffraction method that even a regular 

 swell has components spread over an angle of 

 30° ; for an irregular sea the angle would be 



greater. The rapidity with which the micro- 

 seism amplitude is built up may be explained 

 by the fact that, if once the original progres- 

 sive system of waves is established (from which 

 no microseisms would be expected), only a 

 small amount of wave energy travelling in the 

 reverse direction would be sufficient to produce 

 the necessary pressure fluctuations. Data at 

 present available for the rate of growth of 

 waves under a wind refers to waves growing 

 gradually under a following wind ; it is quite 

 conceivable that the rate of growth of waves 

 travelling downwind, but in the presence of an 

 opposing swell, is greater, on account of the 

 roughness of the sea surface. Observations of 

 the rate of growth should be obtained. Con- 

 trolled experiments could also be made on a 

 smaller scale, using a laboratory wave tank 

 and an opposing artificial wind. 



(3) The amount of wave reflection from 

 the New England coast is probably very small, 

 since the shore in most places is not steep. 

 The exact value of the reflection coefficient can- 

 not be assumed to be the same as for laboratory 

 experiments with a beach of the same slope, 

 since the scaling, for waves of different period, 

 is uncertain ; also in the laboratory experiments 

 the motion was laminar, while in the sea tur- 

 bulence may play a part in the energy dissipa- 

 tion. However, it may be possible actually to 

 determine the extent of reflection from differ- 

 ent parts of a coast by a comparative study of 

 the spectra of pressure fluctuations on the bot- 

 tom, just offshore. 



(Bath pointed out that on the Norway 

 coast the effect is when the front crosses the 

 coast and not the edge of the shelf. After 

 Haskell's formal discussion, Melton asked if the 

 sudden increase in microseisms and the Lon- 

 guet-Higgins theory may not be consistent due 

 to the reversal of winds. Longuet-Higgins 

 pointed out the waves will then be short. Donn 

 pointed out cases where the sea has been calm 

 but there are microseisms. Byerly commented 

 on the fact that at least one seismologist be- 

 lieves microseisms result from winds against 

 mountains. Gutenberg replied to this that be- 

 cause of the location the wind may actually be 

 on the shore. Press described a swell observed 

 on the New England coast in instances of a 

 large swell and no microseisms. Longuet-Hig- 

 gins blamed this on a low reflection coefficient. 

 Deacon inquired if during some of the swell 

 described by Press, which was of eighteen sec- 

 onds period, there were any nine second micro- 

 seisms, and was told no.) 



