Tides 45 
prevent the tides from moving freely. Such obstacles modify 
the motion of the tide waves to such a great extent that the 
theory derived from the positions of the sun and moon gives 
only very rough approximations of what actually happens. 
The rotation of the earth, temperature, climate, barometric 
pressure, etc., also help to modify the tides. As a result of all 
these disturbances in the tides, the only way to predict them 
accurately is to observe them carefully for long periods of 
time and analyze the tabulated observations mathematically 
by a process called Harmonic Analysis. The results of the 
mathematical examination are then compounded in a tide 
predicting machine which automatically computes and prints 
tables of tides years in advance. Such machines are owned 
and operated by the United States, British, and German 
governments. 
Charts are drawn showing the relative heights of the tides 
for large ocean areas. A line upon the chart may indicate all 
places where the tide is of the same height at the same time. 
This line is called a cotidal line. There are also points in the 
ocean where there are no tides at all, known as amphodromic 
points. For example, in the North Atlantic there are three 
such amphodromic points. One is in the neighborhood of the 
Faeroes; another in the middle of the ocean at about latitude 
54° N.; the third southwest of Puerto Rico. 
EXCEPTIONAL TIDES 
In a few harbors there are exceptionally great tides, which 
are in the main due to peculiarly shaped land masses which 
tend to collect the energy of a tide wave and to channel it up 
a narrow funnel-shaped bay. The Bay of Fundy exhibits tides 
of fifty feet. Other bays show the same effect to a lesser ex- 
