72 Proceedings of the Royal Irish Academy. 



XI. — Obseetatioxs o's the Phexomexa of the Tides as Obsekved at 

 Fleetwood (Laxcashiee), with iLLrsTRATioxs Enoir the Tides of 

 PtATHiiuxLA>' (Co. Do>^egal). By the Rev. James Peaeson, M.A., 

 Cantab., Ex-Scholar, Trinity College, Cambridge. 



[Read, May 14, 1877.] 



Hatixg been for some time engaged in making observations upon the 

 tides at Fleetwood, I wish to place on record some of the theoretical 

 results to which I have been led, as well as to say a few words in 

 reference to a graphical construction which I have found very conve- 

 nient for illustrating the relation between cause and effect in their 

 production. The most elementary knowledge on the subject teaches 

 us that the elevation of the tides is due not only to the action of the 

 moon and sun on the waters immediately below them, but also that 

 this action is extended directly through the solid body of the earth to 

 the waters on the obverse side of it. The effect may in part be con- 

 ceived by supposing two moons, one on each side of the earth, the line 

 joining them always passing through its centre, and each attracting, 

 so as to produce on its own side the tide actually exhibited. The 

 first of these being the real moon — the second, or fictitious one, may 

 be called the " anti-moon." In the very same "way, we must imagine 

 the tides which are produced by the sun to be of the same twofold 

 character. "\Ye are thus introduced to four sets of tides, each, as it 

 will be seen, operating under extremely different circumstances, 

 namely — 1st, a limar tide, swelling under the direct action of the 

 moon ; 2nd, an anti-lunar tide, swelling under the obverse action of 

 the " anti-moon;" 3rd, a solar tide, generated under the dii-ect action 

 of the sun ; and 4th, an anti-solar tide, due to the direct action of the 

 "anti-sun." A little reflection will serve to make it apparent that 

 when the true moon is seen in the northern hemisphere or has north 

 declination, the fictitious "anti-moon" or "anti-sun" is over the 

 southern hemisphere, or has south declination. More than this, it will 

 be seen that as either of these luminaries advances from south to north 

 declination, its fictitious antagonist will recede fi'om north to south 

 declination, and vice versa. But forasmuch as land predominates in 

 the northern, and water in the southern hemisphere, the magnitudes 

 of the tides produced will, from this cause alone, be very different, and 

 when the resulting tides, which are formed of the superimposed tides 

 due to the attractions of the sun and moon, consecutively arrive at our 

 shores, they will display this difference of magnitude very prominently. 

 Now, to this point I wish to call particular attention. Theore- 

 tically, "the action of the moon produces a tidal spheroid ; and the 

 protuberances of this spheroid may be considered as composed of two 

 tidal waves — one following the moon, and the other opposite to this. 

 AVhen the sun and moon are not in the equator, one of these may be 

 called the northern, and the other the southern tidal wave. ]N^ow, 

 places in the northern hemisphere pass nearer to the pole of the 

 northern tidal wave, and places in the southern hemisphere nearer to 

 the pole of the southern ; and hence the tides are alternately greater and 



