SECT xiii.] THE SEMIDIUENAL TIDES. 105 



the meridian at 90 distant it is low water. This great 

 wave, which follows all the motions of the moon as far as 

 the rotation of the earth will permit, is modified by the 

 action of the sun, the effects of whose attraction are in every 

 respect like those produced by the moon, though greatly less 

 in degree. Consequently a similar wave but much smaller 

 raised by the sun tends to follow his motions, which at times 

 combines with the lunar wave, and at others opposes it, ac- 

 cording to the relative positions of the two luminaries ; but, 

 as the lunar waves is only modified a little by the solar, the 

 tides must necessarily happen twice in a day, since the 

 rotation of the earth brings the same point twice under the 

 meridian of the moon in that time, once under the superior 

 and once under the inferior meridian. 



In the semidiurnal tides' there are two phenomena par- 

 ticularly to be distinguished, one occurring twice in a month, 

 and the other twice in a year. 



The first phenomenon is that the tides are much increased 

 in the syzigies, or at the time of new and full moon (N. 153). 

 In both cases the sun and moon are in the same meridian : 

 for when the moon is new they are in conjunction ; and 

 when she is full they are in opposition. In each of these 

 positions, their action is combined to produce the highest or 

 spring tides under that meridian, and the lowest in those 

 points that are 90 distant. It is observed that, the higher 

 the sea rises in full tide, the lower it is in the ebb. The 

 neap tides take place when the moon is in quadrature ; they 

 neither rise so high nor sink so low as the spring tides. 

 The spring tides are much increased when the moon is in 

 perigee, because she is then nearest to the earth. It is 

 evident that the spring tides must happen twice in a month, 

 since in that time the moon is once new and once full. 



The second phenomenon in the tides is the augmentation 

 occurring at the time of the equinoxes when the sun's de- 

 clination (N. 154) is zero, which happens twice every year. 

 The greatest tides take place when a new or full moon 



