TIDES 22y 



24 hours and 52 minutes. At flood tide the water is high, at ebb 

 tide, low. Twice each month, at new moon and at full moon, the 

 tides are exceptionally high, owing to the relative position of sun 

 and moon at these times, when they exert a combined influence of 

 the same character upon the waters. Such tides are called Spring 

 Tides. Twice a month also, at the period of first quarter and last 

 quarter of the moon, the interval between high and low water is at 

 its lowest, since at such times the moon and sun act in contrary 

 direction upon the waters, each tending to neutralize the force of 

 attraction of the other. This constitutes the Neap Tides. 



In the open ocean the rise is estimated to be 2 or 3 feet, but 

 along coasts this is generally greatly increased. This is especially 

 the case where the water is crowded into narrowing bays, as illus- 

 trated by the Bay of Fundy, a funnel sea, with the highest known 

 tides of the world. The mouth of this bay is 48 miles wide and 

 its depth at this point from 70 to no fathoms. The bottom rises 

 at the rate of 4 feet per mile for 145 miles, when the head of the 

 bay is reached. Near the mouth the spring tides vary from 12 to 

 18 feet, while at its head, in Cobequid Bay, the range is as high 

 as 53 feet, that of the neap tide being 31 feet. The tide runs up 

 the Petit-Codiac River from the head of the bay, presenting a more 

 or less perpendicular wall. This is the tidal bore which is seen in a 

 number of rivers, such as the Severn and the Wye of England, the 

 Seine of France, the Hugh of India, and the Tsien-Tang of 

 China, where it is sometimes 25 feet high and very destructive. 

 According to the observations of Captain Moor, i^ million tons 

 of water rushed past a point in the river in a minute. Up river 

 the tides are often felt for a considerable distance. Thus its influ- 

 ence is felt up the Hudson as far as Troy, New York, up the 

 Delaware nearly to Trenton, and 70 miles up the St. John's River 

 in New Brunswick, where it is felt at an elevation of 14 feet above 

 mean sea-level. The salt water does not actually run up the 

 rivers to the distances mentioned, the waters of the Hudson, for 

 example, being fresh above Poughkeepsie. The tidal influence is 

 felt rather in a backing up of the river water, which is of course 

 accompanied by a checking of the current, a condition favoring the 

 deposition of material held in suspension. 



Where tides pass through narrow channels, tidal currents or 

 races are produced, which are generally eft'ective agents in scouring 

 the channels or preventing deposition. Where bars or other obstruc- 

 tions retard the entrance of the tide into a narrow bay or estuary, 

 it may not reach its full height before the setting in of ebb tide, 

 and thus the rise and fall will be less than on the unprotected shore. 



