85 



EFFECT OF OVERTIDES 



155. Since the periods of tlie lunar overtides are one-half, one-third, 

 and one-fourth of the period of the M2 component, they unite with 

 the latter to produce a tidal curve which is distorted from a sinusoidal 

 curve, but is repeated without change in each successive period of that 

 component. The form of the curve resulting from the combination 

 of the M2 and M4 components at Philadelphia is shown in figure 34. 

 At this station the amplitude of the M2 component is 2.367 feet, and of 

 the M4 component 0.368 feet, their epochs being 49° and 7°, respectively. 



+3- 



Lunar Hour«. 



Figure 34.— Resultant of M2 and Mt components at Philadelphia. 



The effect of the overtide in this case in increasing the interval from 

 high water to low water and in decreasing the interval from low water 

 to high water is apparent. 



156. If a high water of the M4 component nearly coincides with a 

 high water of the M2 component, the next high water of the overtide 

 will nearly coincide with the low water of the primary component. 

 The overtide will therefore raise the elevation of both the high and 

 low waters with respect to mean sea level. Similarly if the epochs 

 are such that a low water of the overtide nearly coincides with the 

 high and low waters of the primary component, it will lower both the 

 high and low waters of the resultant tide with respect to mean sea 

 level. In the case illustrated in figure 34, the epochs differ by about 

 45° and the overtide has little effect in altermg the relation of the 

 high and low waters of the resultant with respect to mean sea level. 



