xxi.J THE SUN. 375 



As a matter of fact, the moon, revolving round the earth 

 in a lunar month, comes to the meridian about fifty minutes 

 later every day, and constantly changes its position in rela- 

 tion to the sun. Hence, in the course of every lunar month 

 there are two periods (new and full moon) when the times of 

 solar and lunar high water coincide, and the vertical move- 

 ment of the water is greatest ; and two periods (first and 

 third quarters) when solar high water coincides with lunar 

 low water, and the converse, and when therefore the 

 vertical movement of the water is least. The former are 

 called spring tides, and the latter neap tides} 



In the open sea, the water is raised by the attraction of the 

 moon, or of the moon and sun combined, and then falls ; so 

 that the true tidal wave represents a mere oscillatory move- 

 ment up and down. The lunar wave rises, in the open ocean, 

 to a height of about 2\ feet, and the solar wave, to about 

 one foot. But, in narrow channels, the tidal wave becomes 

 converted into a wave of translation, and the water actually 

 moves backwards and forwards (p. i8o). This was seen to 

 be the case in the tidal part of the Thames, which was referred 

 to in the opening sentence of the first chapter. 



The movement of the water of the Thames at London 

 Bridge, in fact, formed the s.tarting-point of those studies 

 which have gradually expanded into these one-and-twenty 

 chapters. " What is the source of the Thames ? " was the 

 question first proposed for discussion ; but, simple as this 

 question seemed, it could not be answered, even in outline, 

 until this last chapter had been reached ; and something had 



^ It will be understood that only a very general notion of the origin 

 of the tides is attempted to be given here. So complicated a subject is 

 beyond the scope of tliis work. 



