132 A. W. Duff — Secondary Undulations 



of course, tend to set up partial vibrations (as would ine- 

 qualities in the thickness of a cord,} 



The above comparison then, while not at variance with the 

 seiche theory, affords only partial confirmation of it. To 

 other cases no calculation can be applied. Digby is on a basin 

 of irregular form which may have somewhat regular vibrations, 

 though theory is at present inadequate to calculate the period 

 of such irregular bodies of water. The same is true of 

 Halifax, with the peculiarity that the basin is one marked 

 out by a series of banks (Poseway, Le Have, Emerald, &c.) 

 which, while not coming to the surface, bound an irregular 

 bay, attaining at places a depth of 300 fathoms. Pictou and 

 Quaco are also on partially enclosed bays (reckoning the Quaco 

 " ledges " as part of the boundary of the latter). At Grind- 

 stone Island (in the middle of the Gulf) and St. Peter's .Bay 

 (on the outer or Gulf side of P. E. I.) the undulations are 

 always or nearly always irregular, as might be expected of 

 points near the middle of such an irregular body of water as 

 the Gulf of St. Lawrence. 



The case of St. Paul's Island (at which there are periods of 

 4*7 and 12*9 m.) might be thought at first inexplicable, as the 

 island is in Cabot strait between prominent headlands on 

 either side, with no suggestion on an ordinary map of 

 a channel or basin of oscillation. But a glance at the 

 nautical chart (of 1891) shows that, while the main chan- 

 nel has a great depth (approaching 300 fathoms), on both 

 sides of this channel, there is an abrupt change of depth and 

 gradient and the contours of, say, 100 fathoms on opposite 

 sides of this channel run roughly parallel for a distance 

 greater than the width of the strait. Moreover, on the Cape 

 Breton side and adjacent to St. Paul's Island, there is a de- 

 pressed basin, only part of the boundary of which reaches 

 the surface. Thus we have clearly the conditions for second- 

 ary undulations according to the " seiche " theory. 



The case of Yarmouth presents a striking peculiarity. The 

 seiche theory gives a very satisfactory account of the periods 

 at Yarmouth ; but why should the period at high tide be regu- 

 larly twice as great as at low tide ? The mere change in 

 depth of the channel can have no such striking effect. The 

 only other factor that changes with the state ot the tide is the 

 direction of the tidal current. I find myself unable to ex- 

 plain satisfactorily why the reversal of the current should 

 have such a marked effect. A thorough examination and elu- 

 cidation of this peculiar case might lead to a more satisfactory 

 explanation of the whole subject than has yet been offered. 

 It might also be noted that at' St. Paul's Island the uuusual 

 elevation of water level at both high and low tide, on the 





