CnAP. XIV.] THE TIDIES. 150 



is at the hour of 9 a.m., the tidal action of the hmi- 

 soLar force is at its greatest point on the Atlantic, so 

 as to make the undulation or vibration then formed 

 the principal source of the tidal undulations vibrating 

 through the Atlantic until the next action of a tide- 

 raising force. 



Now, when it is 9 a.m. in the central parts of tlie 

 ocean, it is about 12 (noon) about the western shores 

 of Europe. And as on the days of full and new 

 moon, the high tides reach those shores at about 

 4 p.M and 4 a.m.,^ the high tides two days after 

 (when the solar and lunar tides are in conjunction) 

 occur at about 6 a.m. and 6 p.m., low water occurring 

 at the intermediate hours, which are those of noon 

 and midnight : but when it is noon on the western 

 shores of Europe, it is 9 a.m. about the central parts 

 of the ocean. Thus the time of low water occurs at 

 noon on the shores of Europe, because it is then 

 9 a.m. in the central part of the Atlantic, and the 

 meridian of 9 a.m. and 9 p.m. is, approximately, that 

 on which the combined action of the sun and moon 

 tends to raise the high spring-tides about two days 

 after full and new moon. When the lunar and solar 

 forces which raised the tide in the centre of the 

 ocean are withdrawn, it falls in the centre of the 

 ocean, and rises on the shores. And since the hours 

 of 9 a.m. and 9 p.m. are the times of hio-h water in 

 the centre of the ocean, the intermediate hours 

 (which are those of 3 p.m. and 3 a.m.) are the hours 



' P. 151. 



