528 THE POPULAR SCIENCE MONTHLY 



these ocean basins to send off progressive waves into other parts of the 

 oceans and into seas, gulfs, bays and tidal rivers. 



By a stationary oscillation is meant a mode of motion which can be 

 readily set up and maintained in a tank, vase or other artificial vessel 

 of water. High water at one end of a rectangular body of water occurs 

 when it is low water at the other end, if the simplest mode of oscilla- 

 tion be under consideration. Between the two ends is a line, styled 

 " nodal line," along which there is neither rise nor fall but across which 

 the horizontal motion of the liquid particles is comparatively great. 

 In order that a large and regular oscillation may be maintained, it is 

 necessary that the natural period of the basin of water be very nearly 

 equal to the period of the applied forces : just as a resonator must have 

 certain dimensions if a particular musical tone is to be reinforced by 

 its presence. An oscillation is best sustained if the phases (or time- 

 angles) of the forces, all parts of the system being considered, agree, 

 as well as may be, with the phases of the velocities of the water par- 

 ticles. This furnishes a clue to the times of the tides when one knows 

 the times of the vanishing of the forces. 



Obviously, if two rectangular basins performing simultaneous oscil- 

 lations in accordance with their simplest modes be put together end-to- 

 end, and the partition between them removed, the whole body may be 

 made to so oscillate that high water will occur simultaneously at the 

 two ends, while it will be low water over the central portion. The 

 nodal lines will remain in the same positions as before, crossing the 

 individual bodies midway between their real and virtual ends., or the 

 new body at points one quarter of the body's length from either end. 

 The individual bodies each comprise a half wave-length of the whole- 

 wave system to which they now belong. The oscillation in each of the 

 individual bodies is said to be " uninodal " and that in the whole body, 

 " binodal." 



No-tide Points 



Points shown upon the charts (Figs. 5, 6 and 7) from which the 

 cotidal lines radiate, are no-tide points ; that is, points at each of which 

 there is no rise and fall of tide. In the oceans these points are due to 

 the fact that the times of the tides around them and which times are 

 dependent upon stationary or progressive waves, or both, must take 

 successively all values from one to twelve, because in open water sud- 

 den changes in time can not occur. 



In narrow arms of the sea, no-tide points may result from dependent 

 stationary oscillations influenced by the deflecting force of the earth's 

 rotation. This, for the northern hemisphere, is such that if we always 

 face the direction towards which the flood or ebb stream is flowing, 

 water will be piled up upon the side of the channel then situated upon 

 the right, and drawn away from the opposite bank. The reverse of 

 this occurs for channels situated in south latitude. 



