i;6 POPULAR LECTURES AND ADDRESSES. 



ist to December 3ist, 1877, and in Broad Sound, 

 Queensland, Australia, from July I5th, 1877, to 

 July 23rd, 1878. Look at this one of these 

 diagrams/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 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 between 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-pre- 

 dicting machine. At a certain time the two 

 high waters become equalised, and the two low 

 waters very unequal (see p. 172 for real examples). 

 The object of the harmonic analysis is to analyse 



