88 THE SEMIDIURNAL TIDES. SECT. XIII. 



account of their resistance, the rapid motion produced 

 in them by rotation prevents them from assuming at 

 every instant the form which the equilibrium of the 

 forces acting upon them requires. Hence on account 

 of the inertia of the waters, if the tides be considered 

 relatively to the whole earth and open seas, there is a 

 meridian about 30 eastward of the moon, where it is 

 always high water both in the hemisphere where the 

 moon is and in that which is opposite. On the west 

 side of this circle the tide is flowing, on the east it is 

 ebbing, and on every part of 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 J moon, though greatly less in de- 

 gree. 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 op- 

 poses it, according to the relative positions of the two 

 luminaries ; but as the lunar wave 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 

 particularly 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 in- 

 creased in the syzygies, 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 opposi- 

 tion. In each of these positions, their action is com- 

 bined 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, 



