2 SCIENCE PROGRESS 



Tidal Oscillations in the Ocean 



By the range of the tide at a place is meant the excess of the 

 depth of water available at high-water for floating a ship at 

 the place above the corresponding depth at low-water. Until 

 recently measurements of the range of the tide were made only 

 at places close to the coasts. They are, in fact, among the 

 results furnished by the use of a tide-gauge. It was supposed, 

 chiefly on theoretical grounds, that the range of the tide in the 

 open sea was much smaller. For a complete understanding of 

 the tides, it is desirable to ascertain the range in the open 

 ocean and in partially enclosed seas, such as the English 

 Channel, by direct observation. It appears that ordinary 

 methods of sounding are not available for this purpose and new 

 instruments have been specially devised, one by Captain Adolf 

 Mensing of the Imperial German Navy, the other by Admirals 

 Mostyn Field and Purey-Cust. The preliminary results obtained 

 by the use of the instrument due to the latter are very striking, 

 a range of tide in the Channel amounting to no less than 

 24 ft. having been measured at a place about midway between 

 Beachy Head and Dieppe. Systematic observations of this 

 kind may be expected to throw much light on the nature of 

 tidal oscillations. The extent to which the tide wave in the 

 Atlantic Ocean, for instance, is an oscillation generated in that 

 ocean by the direct action of the Sun and Moon, as contrasted 

 with a progressive wave, generated in the Pacific and Southern 

 Oceans and entering the Atlantic betweeen the promontories of 

 South Africa and South America, is in some degree a matter of 

 controversy. The systematic study of the tides in the open 

 ocean, as distinct from the ebb and flow along the coasts, may 

 be expected to go far towards settling the question ; the whole 

 value of the method of cotidal lines, as developed by Airy and 

 Whewell, depends upon the answer that may be obtained. 



Like other natural motions, the tidal oscillations of the ocean, 

 maintained by the attraction of the Sun and Moon, do not take 

 place without friction ; and one effect which such friction can 

 bring about is a steady diminution in the speed of the Earth's 

 rotation. The friction of the tides which were in the past raised 

 by the Earth in the Moon, if the Moon once possessed oceans, 

 may in like manner have operated to diminish the speed of the 

 Moon's rotation ; this may be the reason why the Moon now 



