NEWTON S PRINCIPIA. 413 



Southern Ocean, the tide wave would travel along its 

 length with a velocity proper to its depth. This, as may 

 be supposed, is very irregular. The mean velocity of the 

 tide wave would indicate a mean depth of three and a 

 half miles. 



Again, if we suppose that a tide wave would be re 

 flected from the northern coasts of America, and regard 

 the Atlantic as a gigantic closed gulf leading out of the 

 South Sea, the velocity of the tide wave would not merely 

 depend on the depth. Until we know more of the nature 

 of the bottom of the sea, we cannot determine theore 

 tically the motion of the tide wave. 



By theory, the diurnal tide wave should be very small 

 near the equator and poles. But this is not the case. 

 The reason is manifest. TVe have neglected, in our theory, 

 the effect of the configuration of the coast. The importance 

 of this is manifest from the slight sketch we have given of 

 the progress of the tide over the ocean. 



It is impossible for us to do more than merely allude to 

 the effect of wind, shoals, &c. on the tides. Neither can 

 we enter on the question of aerial tides. That such tides 

 must exist is evident, for the attraction of the luminaries 

 will disturb the air as well as the water. The extent of 

 these oscillations have even been determined by very ac 

 curate observations of the barometer. The variations of 

 pressure, as indicated by the barometer, also affect the 

 ocean tides in a remarkable manner. Daussy, Lubbock, 

 and Birt have investigated the amount, and shown that a 

 rise of one inch of barometer will cause a depression of 

 the tides of 7 inches in London, 11 at Liverpool, and 13 

 at Bristol. 



