TIDAL PHENOMENA OF THE ST. JOHN RIVER. 6^ 



ARTICLE IV. 



TIDAL THENOME^^A OF THE ST. JOHN RIVER 

 AT LOW SUMMER LEVEL. 



BY A, WILMER DUFF, BI. A. 



(Read March 2iid, 1897.) 



I. Free and Forced Vibrations. 



The waters of the earth have two somewhat different kinds 

 of motion. There are, first, the steady motions, such as the 

 Gulf Stream, caused ultimately hy the heat which we receive 

 from the sun. Secondly, there are motions of vibration, 

 including waves of various kinds and tides. This second 

 class also admits of an important sub-division. Firstly we 

 have those motions of vibration whose rates are determined 

 merely by the properties of water (especially its mass) and 

 by its weight, or the force which the earth exercises on it ; 

 these motions being analagous to the motion of a pendulum, 

 whose rate is determined by its length and the earth's attract- 

 ion. This kind of vibratory motion we may call the free or 

 natural vibrations of the water. But there is a second class of 

 motions of vibration whose rates are determined l:)y the motions 

 and attractions of bodies beyond the earth, especially the moon 

 and sun. These motions we may call \hQ forced or artificial vibra- 

 tions of water masses. When we speak of tides, we are inclined 

 at first to think of them as merely forced vibrations ; but, in 

 reality, the forced vibrations give rise to free vibrations and the 

 two kinds of vibration are quite inseparably mixed up in tidal 

 phenomena. For instance, the highest authority on tides 

 (Lord Kelvin) regards the tides of the English Channel as mostly 

 a free vibration of the water, see-sawing or teetering about a line 

 passing from Portland to Havre ; and William Ferrel (probably 



