80 



For example, at Fort Hamilton, New York Harbor, M2°=221° 

 and N2°==204°. The parallax age is therefore 



1.837 (221°-204°) = 31 hours. 



Perigean tides occur at this station, therefore, a little more than one 

 day after lunar perigee ; and apogean tides a little more than one day 

 after lunar apogee. 



The parallax age at stations throughout the world ranges up to 

 3 days. In some regions it has a negative value. 



147. Combined effect of S2 and N2 components. — Perigean and apogean 

 tides tend to obscure the spring and neap tides at stations at which 



Figure 31. — Predicted high and low waters at Boston, Mass., January 1937. 



the amplitude of the N2 component exceeds that of the S2 component. 

 A typical monthly variation of high and low water at such a station 

 is shown by the plot, in figure 31, of the predicted tides during January 

 1937, at Boston, Mass., where the amplitude of the N2 component is 

 the larger. These tides may be contrasted with the predicted tides 

 at Sheerness, England, during the same month, shown in figure 32. 



fiOn 



1^- 





'OooOqOOO^'^ 



o °oOoO, 





I cl 



! (2' 



o o 



00 O OOg 





**oo O O O„0«OoO0 

 Z. 2 Q. 



000'*00c 



>o"o"o- 



oo 



,0 O O OqQ 



' I-I A I I t> 



^cuj 



I I I I I 



lO 



15- 



I I 



35" <H 



J L_i I L_l — I I 



FiGTjEE 32.— Predicted high and low waters at Sheerness, England, January 1937. 



At Sheerness the amplitude of the S2 component considerably exceeds 

 that of the N2 component. The figures illustrate quite strikingly 

 the reason why the terms "spring" and "neap" tides are commonly 

 used in England, but not in the United States. 



