306 KELVIN 



1877, to July 23rd, 1878. Look at this one 01 these dia- 

 grams, a diagram of the tides at the north-east corner 

 of Australia. For several days high water always at 

 noon. When the tides are noticeable at all we have 



FIG. 131 Declinational Tide 



high water at noon, and when the tides are not at noon 

 they are so small that they are not taken notice of at all. 

 It thus appears as if the tides were irrespective of the 

 moon, but they are not really so. When we look more 

 closely, it is a full moon if we have a great tide at noon; 

 or else it is new moon. It is at half moon that we have 

 the small tides, and when they are smallest we have 

 high water at six. There is also a great difference be- 

 tween day and night high water; the difference between 

 them is called the diurnal tide. A similar phenomenon 

 is shown on a smaller scale in this curve, drawn by the 

 first tide-predicting machine. At a certain time the two 

 high waters become equalised, and the two low waters 

 very unequal (see p. 304 for real examples). 



The object of the harmonic analysis is to analyse out 

 from the complicated curve traced by the tide gauge the 

 simplest harmonic elements. A simple harmonic motion 

 may be imagined as that of a body which moves simply 

 up and down in a straight line, keeping level with the 

 end of a clock hand, moving uniformly round. The ex- 

 ceedingly complicated motion that we have in the tides 

 is analysed into a series of simple harmonic motions in 

 different periods and with different amplitudes or ranges; 



