SUN, MOON, AND TIDES — COLBERT 189 



compare the characteristics of the tides of these two ports, we note 

 that at Seattle a lower low water is followed by a lower high water, 

 and a higher low is followed by a higher high. At Honolulu, a lower 

 high is followed by a lower low and a higher high is followed by a 

 higher low. 



At San Diego there is a further contrast to each of the other two 

 ports. Here both the high and low waters of a day differ considerably 

 from each other, and the difference between successive high waters 

 about equals that between successive low waters. 



There is another variety of tide for which we may refer to tidal 

 observations made at Galveston, Tex., and, halfway around the world, 

 at Manila in the Philippine Islands for the same 4 days. At both 

 ports two high and two low waters each day gradually shift to one 

 high and one low. There is usually a stand of the tide when the 

 lower high and the higher low waters merge. At this time there is 

 little difference in height. This stand of the tide takes place at 

 Manila on the rising tide and below the mean level, and at Galveston 

 on the falling tide and above that level. 



In the tide-gage records of the foregoing seaports are found the 

 major features of the varieties which occur in the rise and fall of the 

 tide. At whatever place the tide has been observed, certain dis- 

 tinctive features have been found to distinguish the variety of that 

 tide. 



WATER OSCILLATION IN A TANK 



The great physical fact of the tide, apart from the particular 

 characteristics of its rise and fall, is the continual ebb and flow of 

 the sea at a periodic rate, due to the regular movements of the sun 

 and the moon which furnish the attracting forces to the waters of 

 the earth. This fundamental movement of the sea may be illustrated 

 by the action of water in a tank, which has been put in motion by some 

 outside force causing the water to flow to one end. The water level 

 will be raised at that end and will be lowered at the other. Being 

 fluid, the water will not maintain this position, but will flow toward 

 the opposite end. It will oscillate from one end of the tank to the 

 other, the period of oscillation depending upon the length of the tank 

 and the depth of the water. The movement of the water will follow 

 what is known as a stationary wave motion. The greatest change in 

 water level will occur at the ends of the tank, but across the middle 

 there will be no change. This line of no change, or axis, is the "nodal 

 line" of the tank. 



If the movement of water occurred in two tanks placed end to end 

 to form two sections of a combined tank, and the partitions between 

 the two were removed, the water would oscillate or swash about two 

 axes or nodal lines in the combined tank. There would now be three 



