220 POPULAR LECTURES AND ADDRESSES. 



of Good Hope, which Staff-Commander Burdwood 

 showed me in the Hydrographical Office of the 

 Admiralty) valuable for determining this very im- 

 portant element, for ports on all seas where any 

 approach to a knowledge of the laws of the tides 

 has been made. 



To collect information on this point from all 

 parts of the world will be one of the most interest- 

 ing parts of the work of the Committee. 



9. Another very interesting subject for inquiry is 

 the lunar fortnightly, or solar semiannual, tide, the 

 determination of which will form part of the com- 

 plete harmonic reduction of proper observations 

 made for a sufficient time. The amounts of these 

 tides must be very sensible in all places remote 

 from the zero line l of either northern or southern 

 hemisphere ; unless the solid earth yields very 

 sensibly in its figure to the tide-generating force. 2 

 Thus it has been calculated that if the earth were 

 perfectly rigid, the sum of the rise from lowest to 

 highest at TenerifTe, and simultaneous fall from 

 highest to lowest at Iceland, in the lunar fort- 

 nightly tides, would amount to 4*5 inches. The 

 preliminary trials of plans for harmonic reduction 

 referred to below, make it almost certain that 

 hourly observations, continued for a month at two 

 such stations as these, would determine the amount 

 of the fortnightly tide to a fraction of an inch, in 



1 Thomson and Tait's Natural Philosophy, 810. 



2 See Appendix A (3) above. 



