OP THE TIDES IN THE PORT OP LONDON. 



21 



examination ; and it would be important for this, as well as for other purposes, to 

 discuss some large collection of observations for other places than London, in the 

 mode which Mr. Lubbock has applied to the London observations. Such collections 

 are known to exist for Brest and for Liverpool. 



The quantity a in the formula is undoubtedly different at different places. It is 

 what Mr. Lubbock, following Laplace, calls the retard, and depends upon what I 

 have termed the age of the tide. It cannot be determined with certainty or exactness 

 without the use of a large body of observations. Its value at London is 2^, at Brest 

 1^ 12"" ; at Portsmouth it is intermediate between the value at Brest and at London, 

 as we should expect, being about 1** 30"* ; but at Plymouth it is greater than it is at 

 London, which, as Mr. Lubbock observes, is at present a very inexplicable cir- 

 cupistance ; probably to be explained only by the determination of the value of this 

 quantity for several other places. 



3. The Correction for Lunar Parallax. — Mr. Lubbock has classified the tide obser- 

 vations which he has discussed according to the value of the moon's horizontal parallax 

 which existed at the time when the tide occurred, and also according to the hour of 

 the moon's transit, so as to form a table of double entry of the differences from the 

 mean interval : this is Table XVII. in his Memoir of 1831, which I here insert. 



Table showing the Diffe^i'ence in the Interval between the Time of the Moon's Transit 

 and the Time of High Water, and the Mean Interval (Column A. Table III.) for 

 every Minute of the Moon's Horizontal Parallax. 



