ijQ BULLETIN OF THE NATURAL HISTORY SOCIETY, 



the chief authority in America) thinks that the tides of the deep 

 water of the North Atlantic may be an eastward and westward 

 swinging motion, like the " wish-wash " of water in a wash-ljowl. 

 In one case only have we a motion of vibration that belongs to 

 one only of these two classes, namely, the case of moderate sized 

 lakes ; for their motions are nearly altogether free vibrations, 

 and it is only in very great lakes that forced vibrations can l)e 

 discovered; for instance, the tides of Lake Michigan onl}' amount 

 to between two and three inches.* 



II. Tides ix Rivers. 



Remembering this distinction between free and forced vibra- 

 tions, let us apply it to the case of a river. Are there any forced 

 vibrations in rivers ; that is, any motions produced by the direct 

 attractions of sun or moon on the waters of the rivers ? Reason 

 will be adduced later for believing that, in the case of the St. 

 John river at least, there is nothing such ; but it cannot be 

 ■denied that in the case of a very large river like the Amazon, 

 whose course is directly east and west, there may be such a dii'ect 

 forced vibration. But there is in most rivers that enter the 

 ocean a secondary forced vibration ; that is, a fluctuation of the 

 level of their waters produced by a periodical rise and fall of the 

 level of the ocean at the mouth. This distinction is sometimes 

 put in this form, that there is no true tide in the St. Jolin rivei', 

 only a " backing-up ; " but such a way of putting it is hardly 

 justifiable. It is true that as the level of the water at the mouth 

 rises, the speed of the stream must decrease, and as there is still 

 practically the same supply of water from the parts of the river 

 farther up, the level must in consequence rise progressiveh' up 

 stream. This is what is meant by a "backing-up." But there 

 is also a flow of salt water up stream for a considerable distance 

 from the mouth, a flow that differs in no respect froni the flow of 

 water up the Bay with the incoming tide. Now we have seen 

 that we cannot limit the word tide to direct forced vibrations 



*T. D. Graham, Vol. xiv., A. A. A. S., 1860. 



