180 ROYAL SOCIETY OF CANADA 



The general causes which produce currents in such regions are: (1) 

 Tidal influence; and (2) a cause of a wider character which shows itself 

 as a tendency in the current to set in some dominant direction. If the 

 latter currents are termed "^constant" it must be understood that they are 

 always subject to tidal fluctuation which may check or even reverse them; 

 ior any general circulation of the water can only be traced as a greater 

 movement in some dominant direction rather than in other directions, 

 when long averages are taken. 



These causes give rise to the normal movements and upon them the 

 influence of the wind and barometer are superimposed as distributing 

 causes. The problem in general is to determine the normal or undis- 

 turbed movement of the water, and to attribute any deviation from the 

 normal to wind influence. There are various methods by which this 

 problem may be approaclied, which we will endeavour to explain. 



Tide, current, wind and barometer. — It may be advisable, before 

 going further, to explain as clearly as may be, the inter-relations of tide, 

 current, wind and barometer. Our fullest knowledge relates to the dis- 

 turbance in the height of the tide. The above report of the British As- 

 sociation committee refers exclusively to disturbance in height. It is 

 also erroneous to suppose that an abnormally high tide is followed by an 

 unusually low tide. The investigations of the Dutch Engineers on the 

 coast of Holland indicate that the effect of gales on the tides is to raise 

 both the low and high water level. This is illustrated by diagrams in 

 the report referred to; and it is abundantly corroborated by our Cana- 

 dian tide curves from recprding gauges. Not only so, but the time of 

 high and low water usually remain unaffected, although the whole tide 

 curve maj' be two or three feet above or below its normal position. 



There is thus little if any support for the assumption that if the tide 

 is raised or lowered by the wind, there will be a corresponding increase 

 in the strength of the tidal streams. For the range of the tide 

 remains practically unaffected, as in reality it is the mean level of 

 the sea which is temporarily disturbed. The rise and fall of the tide ap- 

 pears to be caused by a deep pulsation which extends throughout the 

 whole depth of the water in straits and channels, however great that 

 depth may be. There is strong corroborative evidence for this view from 

 several sources. It is also fairly certain that difference of barometric 

 pressure causes a flow which extends throughout the whole depth of the 

 water. In contrast with this, the wind must necessarily act directly on 

 the surface of the water. 



It thus appears that any disturbance of the usual movement of the 

 body of the water as a whole, is more closely related to difference of baro- 

 metric pressure; whereas wind disturbance necessarily begins on the 



