Pearson — On the Computation of Tides at Fleetwood. 179 



XXX. COMPXTTATION OF TiDES AT FLEETWOOD. DlSCITSSIOIir OP CoR- 



EESPONDING- PHENOMENA IN DTFEEEENT YeAES, ETC. ^^J JaMES 



Peaeson, M. a., Ex-Scholar (lotli Wrangler), Trinity College, 

 Cambridge. 



[Eead, November 11, 1878.] 



In my first' communication on the subject of the Tides, I gave a brief 

 account of the guiding principles which led me to the construction of 

 reyised Tables for their computation, and the graphic process by which 

 I obtained a clue to them. My second^ Paper showed the results of 

 theory as compared with observation, and proved that the course of 

 the "diurnal inequality" was correctly explained and traced out. 

 The object of my present Paper is to illustrate the correspondence of 

 tides in different years, having the same constituents approximately^ 

 If a velocity of angular rotation, equal and opposite to that of the 

 earth, be supposed to be communicated to the moon, the earth will be 

 reduced to rest, and the moon's path in the sky will be represented by 

 a sort of spherical helix, the convolutions of which will ascend from 

 east to west, whilst the moon's declination passes from south to north ; 

 and during that interval the path of the "anti-moon" will be also 

 represented by a corresponding spherical helix, but differing from the 

 former in this respect, that its convolutions will descend from east to 

 west, whilst the actual moon's declination has the fore-mentioned 

 changes. To use a familiar illustration — in the one case we shall have 

 a left-handed cork-screw motion, and in the other a right-handed one. 

 Consequently, the lunar and anti-lunar tides are essentially different 

 in their operation, and so are the solar and anti-solar tides. On the 

 method which I have adopted in dealing with the subject, any single 

 tide during any year will only have one tide corresponding to it, out 

 of all the tides in any other year, and hence it becomes interesting to 

 inquire how far such co-ordinate tides (as I may term them) agree 

 with observation as years advance ; and with this purpose in view, 

 I will select a series of such tides, commencing with the year 1873, 

 and occurring in the same month (April) in each year subsequent ; 

 also another series occurring in the same month (September) in each 

 year. The results will speak for themselves ; but I would invite 

 attention to the tides for 1872, Sept. 15, evening, and 1873, Sept. 5, 

 evening. These have almost identical constituents ; and yet, from 

 atmospheric causes, we have in one case a resulting tide three inches 

 above calculation, and in the other a resulting tide eleven inches 

 below. 



^ Vide ante, page 72. 2 yi^e ante, page 111. 



K. I. A. PKOC, SEE. II., VOL. III. — SCIENCE. P 



