I05 



wave at that instant. Such a line is called a co-tidal line. The first 

 person who attempted to draw these lines was the late Sir John 

 Lubbock; but those given in physical maps are due to the researches 

 of Dr. \\'hewell. 



The co-tidal lines are drawn to represent the position of the tide 

 wave at each successive hour on the day of new or full moon. 



Were the ocean such as we supposed when considering the 

 equilibrium theory, the ridge of the tide wave would be a meridional 

 line, travelling westward round the earth in 24 lunar hours, and 

 accompanied by another at 12 hours' distance. There would thus be 

 24 co-tidal lines, resembling parallels of latitude. 



In the real ocean the forms of these co-tidal lines are greatly 

 distorted by the modifying action of the land. 



The diurnal tide is found to exist from the Scilly Isles to Portland 

 Bill, but apparently not further east. This shows, in all probability, 

 that the tides in the upper parts of the Channel are compound tides 

 due to the interference of two tides differing by a period of twelve 

 hours, so that the diurnal elevation due to the one is balanced by the 

 depression due to the other. The inbend between Beachy Head and 

 the Isle of Wight appears to have day tides which run up to 

 Portsmouth. The range of the tide, that is, the difference between its 

 heights at low and high water, is 12 or 13 feet from Devonshire to 

 Selsea, iS or 19ft. at Brighton, 21ft. at Eastbourne, Dungeness, 

 and Dover. 



Among the authorities on the subject are Newton's Principia, 

 Laplace's Systcme du Monde and Mecanique C^feste, Young in the 

 Enc. Brit, Airy in the Enc. Metrop., and Sir W. Thompson's reports 

 to the Brit. Assoc, &c., the papers of Lubbock and Whewell in the 

 Phil. Trans. Other references are Mrs. Somer\'ille's Mechanism of 

 the Hea\ens and Connexion of the Physical Sciences, Thompson 

 and Tait's Natural Philosophy, vol. I. There is an historical notice 

 by Leslie Ellis prefixed to the De Fluxu et Refluxu Maris of Bacon 

 in Spedding's Edition. 



