334 Mr. Lubbock on the Tides on the Coast of Great Britain. 



water at the south-western extremity of Ireland at about 

 5 o'clock on full and change, arrives at Calais about six hours 

 afterwards, nearly at the same time that another branch, which 

 proceeds round the western coasts of Ireland and Scotland, 

 makes high water at the Orkney Islands, as may be seen in the 

 map. 



It is usual to give the time of high water at foil and change, 

 which is called the establishment of the port. This is marked 

 in Roman figures in the accompanying map. 



The time of high water at full and change is affected by the 

 moon's parallax and declination : if therefore the time be 

 observed at this point of the moon's age, the correction for 

 parallax and declination should be applied with a contrary 

 sign, in order to reduce it to some fixed standard. It would 

 be better to give the time of high water when the moon passes 

 the meridian at 2 o'clock, the time being then, according to 

 theory, not so sensibly affected by parallax. It is also usual 

 to give the height of spring tides and neap tides. These vague 

 terms may answer the purpose of a popular explanation ; but 

 when accuracy is required, the age of the moon corresponding 

 to the given height should be defined by the time of her transit 

 to the nearest minute, and her parallax and declination as well 

 as the month of the year should be stated, which circumstances 

 have a material influence. When but few observations are 

 made, the direction of the wind should also be remarked. 



In order to determine the establishment of any port, the 

 time of high water may be observed at different syzygies, and 

 the mean of these observed times compared with the mean of 

 the moon's corresponding transits. Suppose, for example, the 

 mean of different observed times of high water at any port is 

 five hours, and the mean of the moon's corresponding transits 

 O h 10 m , the establishment of the port is 4? h 50 ; neglecting 

 the influence of parallax and declination. 



Many details concerning the tides on the coast of Great 

 Britain may be found in the following works : Directions for 

 navigating in the North Sea, by M. J. F. Dessiou. Direc- 

 tions for navigating throughout the English Channel, by the 

 same Author. And, Sailing Directions for St. George's Chan- 

 nel, by Messrs. T. and A. Walker. 



For the reasons stated above, the times of high water at 

 full and change given in those works can only be considered 

 as verv rough approximations, until the methods by which 

 they have been determined are explained in detail. 



The lines in the map stretching from coast to coast, are 

 intended to represent the crest of the water, or what may be 

 called cotidal lines, being the series of points where it is high 



water 



