136 A. W. Duff — Secondary Undulations 



Mr. Denison, however, states as the first of the ten " chief 

 points deduced" by him (but without any indication of the 

 details of the evidence), " that the undulations are due to the 

 direct action of atmospheric waves upon the surface of 

 the water at stations [italics mine] and not to ground swells, 

 due to distant storms, or 'seiche' movements, as found upon 

 lakes during atmospheric disturbances." Now with the for- 

 mer or positive part of this conclusion the evidence I have 

 adduced above does not seem at all compatible ; the latter 

 or negative part, denying any other agency, can only mean 

 the sufficiency of the theory of atmospheric waves to account 

 for the period as well as the origin of the secondary undu- 

 lations, which again is wholly at variance with the fact 

 that the barometric disturbances show no periodicity and the. 

 water indulations continue regular and periodic after the baro- 

 metric record has become quite smooth. The second point 

 enumerated by Mr. Denison is that "there is a marked rela- 

 tive correspondence in amplitude between the barometric and 

 water undulations." This I also have found to be the case, 

 provided it be not understood to imply that the two kinds of 

 undulations always (or even usually) occur or continue at the 

 same time. (I would also take some exception to the word 

 si marked.") 



From the above statements in Mr. Denison's paper it has 

 been inferred by some readers that he found regular atmos- 

 pheric oscillations coinciding with and accounting for the 

 water undulations both as regards origin and period. Mr. 

 Denison does not directly say so and this cannot be his mean- 

 ing. There is absolutely no evidence for it in the cases I have 

 referred to above, covering a whole summer of St. John 

 records. How in fact is it conceivable that atmospheric bil- 

 lows many miles in length and moving with a velocity of from 

 60 to 112 miles per hour, should always have a periodicity of 

 half a minute at the mouth of the St. John River — 41 m. at 

 St. John Harbour (less than a mile away) — 12J m. at Quaco 

 (20 miles away) — 69 m. at Yarmouth at high tide, but 30 m. at 

 low tide — 20 m. at Halifax on some occasions, but 33 m. on 

 other occasions and occasionally probably both — 47 m. at St. 

 Paul's Island — 29 m. at Picton, etc. ? 



Effect of Contraction of Bay. 



Reference must be made to another point in the '■ atmos- 

 pheric billows" theory. To account for such slight variations 

 of pressure causing such large variations of water level, it is 

 necessary to suppose that, as in the case of ordinary tides, 

 " minute undulations as they move farther into the bay become 

 magnified as they reach, narrower and shallower portions." 



