22 THE TIDES. 



and falling from m to n and from o to /, and the interval 

 olm is less than mno. Hence the tide is lowest at n and 

 not so low at /, and it is high water at m and o. Hence 

 we have a diurnal tide in addition to the semi-diurnal, 

 this diurnal tide becoming of more importance as we recede 

 from the equator until the co-latitude = moon's declination, 

 when the semi-diurnal tide disappears. 



At the equator the meridional component acts during half 

 a rotation towards the equator, and during the other half 

 from it, and in each case is an elevating force, which, as 

 before, has its greatest effect 90 from the moon. At all 

 places whose latitude is not greater than the moon's declina- 

 tion there is a permanent accumulation. In the circle abed 

 this component is directed towards the north at a and 

 towards the south at c, the points of change being where 

 the great circles from M touch abed. This gives rise to 

 a north and south oscillation. The southerly force being 

 the greater, there will be a residual depression of the water 

 in this region. The depressing force, however, varies, 

 being greatest at a and at c* while the elevating force 

 is greatest where the great circle tangents from M 

 meet the circle. Hence, by 4 and 5 the tide will b& 

 lowest at the latter points and high at the former, and 

 there will be a diurnal tide, as in the former case. Com- 

 bining this with the former result, the effect of both 

 components together will be to give high water at a. 



* If the moon's declination were greater than 22 30', c might be less than 

 45 from M, in which case the force there would be an elevating one. 

 Again, at a place whose latitude was greater than 22 30', and less than the 

 moon's declination, the moon's least nadir distance (= IN] would be greater 

 than 45, and the force depressing. 



