212 On the Tides in the great JYorth American Lukes. 



deep arm of the Lake. The extent of Lake Superior is not equal 

 in length to tlie course here described, and that lake, excepting the 

 projection of Keevvuna Point, presents but few littoral features which 

 would have any sensible influence on the elevation of a tide-wave. 



But it must be borne in rnind, in reference to this subject, that the 

 planetary attraction, on reaching the eastern point of these lakes, 

 having brought with it no " wave," has there to begin with an initial 

 force, and that it must pass over a considerable pordon of the water 

 before its operation can be felt. We cannot say at what distance 

 from the eastern shore this point of sensible effect would be found; 

 but, if Lake Huron were an isolated lake, we should probably look 

 for no lift of the surface, from this cause, even at the western side« 

 The tide-v>^ave, therefore, when it arrives at the Straits of Mackina, 

 is, notwithstanding the favoring approximation of the two shores, 

 probably nearly or <^uiite insensible. It is well known that, although 

 currents and counter currents have been long noticed in these straits, 

 no one has ever regarded them as possessing any of the character- 

 istics of a lunar tide. Even the fact stated by Charlevoix, and to 

 which Mr. Schoolcraft alludes in his travels, ©f his boat floadng one 

 way while the wind blew the reverse, may be sadsfactorilj;- explained. 

 A continuous and strong wind prevailing either way through the straits, 

 will propel so much water out of one lake into the other, as to destroy 

 the equilibration of surface ; when tlie refluent tendency of the ac- 

 cumulated mass will produce a counter current, though the wind 

 Kiay remain unchanged and unabated. Hence Charlevoix's boat 

 may have been "carried against a head wind." 



If then it be probable that there is no sensible dde at the Straits of 

 Mackina, Lake Michigan, including Green Bay, must be considered 

 as deriving little or na assistance, in forming its tide-wave, from the 

 sister lake. That it would exhibit this phenomenon, if it stood alone, 

 few would be inclined to believe, notwithstanding all auxiliary cir- 

 cumstances, of the chain of islands, and the tunnel form of the bay. 

 Indeed, Lake Michigan, though favorable for the increase of a wave 

 sent into it from Lake Huron, yet, from its comparative shallowness 

 and diminuUve breadth, seems unfavorable to the formation of one 

 on its own bosom. 



It is not to be assumed that planetary influences are wholly inoper- 

 auve OH the lake waters. They undoubtedly have their due effect. 

 But that effect is probably nearly or quite insensible. If a calm could 

 he supposed to prevail on the lakes of a sufficient continuance to allow 



