SUN, MOON, AND TIDES — COLBERT 195 



coast of Mexico and less than 1 foot in the Caroline Islands near 

 another nodal line. 



It was noted previously that the tides were small in the Caribbean 

 and in the Gulf of Mexico. At the Atlantic entrance to the Panama 

 Canal, near Colon, there is a range of 1 foot. At Balboa, on the 

 Pacific entrance, there is a considerable rise and fall, amounting to 

 12 to 16 feet. This large range is partly accounted for by the geo- 

 graphic location of this entrance at the end of a loop in the basin, 

 where the highest level is reached by the oscillation of the waters. 



In the vicinity of the Aleutian Islands, there is practically no 

 semidaily tide. This archipelago, extending in an arc over 900 miles 

 from the Alaskan Peninsula, separates this Pacific basin from the 

 Bering Sea basin. The waters in each basin oscillate under different 

 influences of the sun and moon. When differences in elevation are 

 built up between the Pacific side and the Bering Sea side of these 

 islands, the passes between them become what has been termed "spill- 

 ways." Tidal heights in the passes are not great, but the currents 

 are strong. Flood currents frequently flow for 18 hours at the rate 

 of 4 knots. The ebb, flowing southerly, is of shorter duration but 

 exceeds 6 knots, with velocities as high as 9 knots in some of the nar- 

 rower and shoaler passes. Under these conditions, heavy tide rips 

 occur. Some of these tide rips are 8 feet high and could seriously 

 endanger a small ship. 



In speaking of turbulent tidal waters, another type should be men- 

 tioned, namely the tidal bore. This is caused wherever the shore and 

 bottom characteristics restrict the rise of the tide until a substantial 

 difference in elevation is built up. Frequently a wall of water will 

 advance across the shallow area at considerable initial velocity. Dur- 

 ing the survey of Cook Inlet, which I mentioned earlier, we had 

 occasion to observe this tidal bore in Turnagain Arm. This bore 

 could capsize or swamp a small boat. It was necessary to take pre- 

 cautions before the turn of the tide to insure that all working parties 

 and boats were clear of the flats and shoals covered by the bore. The 

 tidal bore in the Bay of Fundy is well known. 



COTIDAL LINES 



In speaking of the principal tidal basins in the oceans, mention was 

 made of some of the locations where high water occurred at the height 

 of the semidaily movement of the stationary wave, induced by the 

 oscillation of the waters in various sections. The times of high water 

 were related to periods following the time of the moon's transit of the 

 meridian of Greenwich. The various locations of high water, as 

 produced by all tidal forces, have been delineated by cotidal lines. In 

 figure 6 there are shown the cotidal lines the Atlantic Ocean from the 



