260 POPULAR LECTURES AND ADDRESSES. 



uring the absolute value of the disturbance 

 produced on terrestrial gravity by the lunar and 

 solar tide-generating motive. But observations of 

 the diurnal and semi-diurnal tides in the ocean do 

 .not (as they would on smaller and quicker levels) 

 suffice for this purpose, because their amounts 

 differ enormously from the equilibrium-values on 

 account of the smallness of their periods in com- 

 parison with the periods of any of the grave enough 

 modes of free vibration of the ocean as a whole. 

 On the other hand, the lunar fortnightly declina- 

 tion al and the lunar monthly elliptic and the solar 

 semiannual and annual elliptic tides have their 

 periods so long that their amounts must certainly 

 be very approximately equal to the equilibrium 

 values. lUit there are large annual and semi- 

 annual changes of sea-level, probably both differ- 

 ential, on account of wind and differences ot 

 barometric pressure and differences of temperature 

 of the water, and absolutely depending on rain- 

 fall and the melting away of snow and return 

 evaporation, which altogether swamp the small 

 'annual and annual tides due to the sun's 



