_Tides._--The waters of the ocean are subject to wave-like undulations, caused by the attraction of the moon and sun, termed the tides. Every day, at the average interval of twelve hours and fifty-one minutes, the "tide rises," and with equal regularity intermediate between these periods it "falls." This rise and fall of the waters along the coast, accompanied frequently by strong currents, are produced directly by the arrival in the shoal water of a pulsation of the ocean, which becomes a true onward-moving gravity wave as it nears the land. In the open sea the amplitude of the tidal undulations is but two or three feet, and their rate of travel in general 700 to 800 miles per hour. On reaching shoal water, however, the onward movement is decreased by friction on the bottom, the waves become higher, and when they meet an outward-flowing bottom current, their bases are still more retarded and the slope of their fronts increases until the water falls forward and breaks into foam. On the Atlantic coast, each tidal wave reaches the land broadside on, as it were (Fig. 4), and at the outer capes high water occurs at practically the same time from Florida to New England, but its farther landward progression is greatly modified by the shape of the coast and the depth of water in the estuaries and other indentations. When the wave as it rushes landward enters a broad water body through a narrow entrance, as the Gulf of Mexico, for example, it spreads, and as the impulse is transmitted to larger and larger volumes of water, it decreases in height. (In a critical study the tides originating in the Gulf itself should be considered.) At Galveston, Texas, the mean range between high and low tide is less than one foot. When, however, an estuary with a broad mouth receives a tidal wave from the ocean, the impulse is more and more concentrated and the wave rises higher. At the head of the Bay of Fundy the difference between high and low water is from 50 to 60 feet. For the reason just stated, the tidal wave is generally higher in the Atlantic coast estuaries than on the ocean capes, and under favourable conditions may be transmitted for long distances up the rivers emptying into such estuaries, and may be felt where the mean elevation of the stream is several feet above the mean level of the sea on the neighbouring open coast. Tidal waves pass up the Hudson to Troy, a distance of 150 miles from the Narrows, where the mean range is 2.3 feet. In St. John River, New Brunswick, the tidal impulse is felt at Frederickton, 70 miles from the Bay of Fundy, and at an elevation of 14 feet above its surface. In the St. Lawrence estuary and river the tidal waves ascend 283 miles to Three Rivers, a few miles below Montreal, where the mean elevation is about 11 feet and the mean range of the tide 0.9 foot. In the Columbia the range of the tide is about 6 inches at a distance of 140 miles from the ocean.