Pearson — On the Comimiation of Tides at Fleetwood. Ill 



XIX. — CoMPrrTATioN OF Tldes at Fleetwood. — Eesults of Theoet 

 AND Obseetatiojst. By James Peahson, M. A., Ex-Scholar (ISth 

 Wrangler), Trinity College, Cambridge. 



[Eead, November 12, 1877.] 



In resuming the consideration of the subject of the tides, it is not my 

 intention to enter at any greater length into the theory which has pro- 

 duced results so closely ia accordance with observation : it is sufficient 

 for me to make some remarks on the principal cause of such discrepan- 

 cies as are found to arise where tables have been used which are based 

 upon that theory — and amongst these disturbing influences, the pres- 

 sure of the atmosphere ranks foremost. A very simple process may be 

 employed to establish this. It is found that, in the same month, but 

 in different years, the same, or nearly the same, constituents have to 

 be employed in computing a tide ; and as like causes produce like 

 effects in nature, the resulting tide ought to be the same in both cases. 

 Eut it is not so, and the variation is found to depend on the height of 

 the mercury in the barometer, or, more correctly, on the magnitude 

 and direction of the gradients indicated in the weather reports issued 

 in the newspapers. Another disturbing element is that which depends 

 on the suddenness with which the pressure shifts its direction. Thus, 

 a south-east wind rapidly changing into a south-west wind causes an 

 unusual elevation of the level of the Irish Sea. Two examples may 

 serve by way of illustration. In the first case there are exhibited 

 two tides which have very nearly the same constituents, and which, 

 in consequence of the atmospheric conditions being the same, give re- 

 sults both agreeing with observation. The heights are given in feet 

 and inches. 



Case I. 



Moon's Transit, B, 

 Corr. for Anti-lunar, . 

 Moon's Hor. Parallax, 

 Anti-lunar Declination, 

 Solar Declination, 



1876, Sept. 3, Morning Tide, 

 Anti-lunar and Solar. 



39 



.54' 

 18° 



ft. in. 



25 5 



+ 1 



45" - 7 



12']Sr.des. -7 



8° N. +4 



1877, Aug. 23, Morning Tide, 

 Anti-lunar and Solar. 



24 8 obs, 24 8 



Bar. 29 9. No wind. 



d. 

 21 



54' 



20° 



35 



ft. 

 25 



4 

 + 1 

 3" - 11 



41'N. des. -10 



12° N. -I- 4 



24 obs. 24 



Bar. 29-8. No wind. 



