188 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



appearance with regard to high or low water. In the upper reaches 

 of tidal rivers, however, especially where there is considerable fresh- 

 water discharge, this is not the case. Thus at Albany, near the head 

 of tide water on the Hudson River, at times of freshets the tide may 

 rise for 3 or 4 hours and fall for 8 or 9 hours. This gives the tide 

 curve in such rivers a characteristic appearance, the rise being repre- 

 sented by a short steep line, while the fall is represented by a longer 

 gently-sloping line. 



This feature in river tides is obviously due to the resistance of 

 the river bottom and banks to the upstream progress of the tide. 

 Furthermore, the drainage waters which find their way into a river 

 give rise to a current that tends to flow downstream constantly. This 

 acts as an added element of resistance to the progress of the tide 

 upstream. And thus the duration of rise of tide is shortened, while 

 the duration of fall is correspondingly lengthened. 



In certain rivers the tide during a portion of its rise comes as a 

 wall of water several feet in height. This phenomenon is known as 

 a bore and is found to occur in the upper regions of tidal rivers 

 having large ranges of tide, the channels of which are obstructed 

 by bars and mud flats. This may be considered as the limiting case 

 of river tides, in which the steepness of the rising tide becomes so 

 great as to become vertical during a part of the rise. In North 

 America the best known bore is that occurring in the Petitcodiac 

 River in Canada. The largest bore is probably that found in the 

 Tsientang, a Chinese river which empties into the China Sea. In 

 this river the bore comes as a wall of water 10 feet or more in height, 

 its front a sloping cascade of bubbling foam. 



Throughout the world, with but rare exception, the sovereignty 

 of the moon over the tide is clearly exhibited by the retardation in 

 the times of high and low water by about 50 minutes each day. 

 Thus in figure 1 it is seen that the first high water of May 28 at 

 Norfolk occurred at 6 o'clock and that each day thereafter it came 

 about an hour later. The other high water and also the low waters 

 are seen to have occurred approximately an hour later each day. 

 And at Seattle, with a totally different kind of tide, a similar re- 

 tardation in the times of high and low water is seen to have occurred. 

 This merely confirms the old adage that " the tide follows the moon." 

 For the transit of the moon over any place occurs each day later by 

 50 minutes, on the average. 



There are some places, however, where the tide appears to follow 

 the sun rather than the moon. That is, instead of coming later each 

 day by about 50 minutes, the tide comes to high and low water at 

 about the same time day after day. Thus at Tahiti in the Society 

 Islands, it has been known for many years that high water generally 

 comes about noon and midnight and low water about 6 a. m. and 



