TIDAL O B S E 11 V A T I N S 



155 





Lunar-declination efiect on the Half- 



monthly Inequality. 







High water. 



Low water. 



High water. 



Low water. 







Inequality iu 



time. 



Inequality in time. 



Inequality in height. 



Inequality in height. 



C 's tra. 















D = 



+ 8° 



■+16° 



±23° 



D = + S° 



+ 16° 



+ 23° 



D=+8° 



+16° 



+23° 



D=+8° 



+16° 



+23° 



1" 





.. 



linos'" 



.... 



.... 



17" 13" 







... 



21".5 



... 



... 



ir'.8 



... 



3 



10 



»41in 



10 37 



10" 36"' 



16''49"' 



16 48 



16" 47-" 



21M 



21.4 



21f'.3 



1 2'^'. 2 



12.3 



12'''. 2 



5 



10 



40 



10 39 



10 33 



16 46 



16 42 



16 44 



20.0 



20.0 



20.2 



13.4 



13.4 



13.5 



Y 



. . 





11 27 







- - . . 



17 27 







. . - 



19.3 



. . . 







14.2 



. . . 



9 



11 



46 



11 52 



(11 57)? 



18 18 



18 01 



17 52 



20.0 



20.1 



20.2 



12.7 



13.0 



13.2 



11 







11 40 



.... 







17 46 











21.0 











12.2 







Mean, 





11 14 







17 m 







20.6 





12.8 





From the above compilation we can infer tliat for increasing declination the ?ion- 

 periodical part of the half-monthly inequality decreases ; this applies to the times of 

 high and of low water ; the total range between 0° and ±26° probably amounts to a 

 few minutes. Respecting the heights, an increase of the moon's declination pro- 

 bably produces a decrease (in the non-periodic part) of the height of high water, and 

 certainly an increase in the height of low water ; the range, therefore, will diminish 

 Avith an increase of declination. The total range between zero and maximum decli- 

 nation probably amounts to a fraction of a foot. 



The periodical and epochal part of the declination effect cannot be investigated 

 on account of an insufficiency of material ; for the same reason we are compelled to 

 omit any discussion of the effect of changes of the solar declination and parallax, 

 which would demand a series of observations extending at least over one year. 



Investigation of the Diurnal Inequality. — The phenomenon of alternate higher and 

 lower high waters and alternate higher and lower low waters, also alternate earlier 

 and later high or low waters, is known as that of the diurnal inequality. Its cycle 

 is a lunar day, and as its magnitude depends on the moon's declination, it goes 

 through its phases in about 14 days, or half a lunation. Generally speaking, and 

 without reference to retard, this inequality vanishes when the moon passes the 

 equator, and reaches its greatest development when the moon attains its greatest 

 north or south declination. The full effect is not generally reached until several 

 days after the moon has passed these positions. The high waters alone may be 

 principally affected, or the low waters alone, or both may exhibit the inequality. 

 Part also of this diurnal tide depends on the sun, and appears therefore in certain 

 months of the year more distinct, and in other months less so. The tidal theories 

 agree in assigning a large diurnal inequ.ality to the middle latitudes, and a small 

 one to equatorial and polar latitudes, the existence of the diurnal inequality in Baffin 

 Bay, along the west coast of Greenland, has long been known to navigators, and by 

 the labors of Dr. Kane it has been traced beyond Smith Strait as far up as latitude 

 78|° N. The present series not only confirms these results but gives us by far the 

 better special knowledge of the various features of the phenomenon. The diurnal 

 inequality experienced in these high latitudes is evidently the result of the propa- 



