42 U. S. COAST AND GEODETIC SURVEY 
1947 1948 
Jan. Apr. July Oct. Jan. Apr. July Oct. Dec. 
1.0 
0.5 
Sea level — —— — — 
Aalt-ideleve 0.0 
Fic. 35.—Monthly sea level and half-tide level, Boston, 1947-48. 
either positive or negative. Hence sea level may be either above or below half-tide 
level, depending upon the phase and amplitude relations. 
Along the Atlantic coast of the United States cos (2M2—Ms)is generally negative, 
while cos (Mz—K:i—O:) is generally positive. Hence along this coast half-tide level 
is below sea level, with but few exceptions. Along the Gulf coast both cosine terms in 
the formula are generally positive, so that here half-tide level may be either above or 
below sea level, depending upon which term has the greater value. On the Pacific 
coast the first cosine term is positive at some places, while at others it is negative; the 
(Ki +0,)? 
M, 
second cosine term, however, is generally negative. Here, however, is, as a 
rule, more than 40 times as great as My, and therefore at most places along this coast 
half-tide level is above sea level. 
The periodic variation in K, and O, from year to year is much greater than in Mg. 
Hence, where the ratio of (K,-+O,;)? to Mp is large, appreciable variations in the relation 
of half-tide level to sea level may be expected from year to year. On the Atlantic 
coast this ratio is small, being at most places about 0.1; but on the Gulf and Pacific 
coasts it is relatively large, being at most places greater than unity. It is therefore 
to be expected that the relation of half-tide level to sea level will differ but little from 
