118 J. E. Hilgard—Tidal Waves and Currents. 
wave thus developed, and decreases in width from its entrance 
toward its head, the tide rises higher from the mouth upward. 
This is due to the concentration of the wave by the approach 
of the shores, and to the gradual shoaling of the bottom. 
is effect is strikingly illustrated by a generalization of the 
heights of the tides on the Atlantic coast of the United States. 
That coast presents, in its general outline, three large bays: the 
Great Southern, from Cape Florida to Cape Hatteras; the Great 
Middle, from Cape Hatteras to Nantucket: and the Great 
Eastern, from Nantucket to Cape Sable, now known as the Gulf 
of Maine. It will be seen that the tide-wave arrives at about 
the same time at the headlands, Cape Florida, Cape Hatteras, 
Nantucket and Cape Sable, and that at those points the height 
is inconsiderable compared with the rise at the head of the sev- 
eral bays. Thus, at Cape Florida the mean rise and fall is only 
one and one-half of a foot; at Hatteras, but two feet; while at 
the intermediate entrance to Savannah it reaches seven feet, de- 
clining in height toward both capes. Again, at the head of the 
Middle bay, in New York harbor, it reaches five feet, while 
on the southeast side of Nantucket Island it is little over one 
oot. The configuration of the Eastern bay is less regular, and 
the correspondence of heights is not so obvious. The recess of 
Massachusetts bay is well marked, the increase in height reach- 
ing ten feet at Boston and Plymouth. Rolling on eastward 
vances into a narrow channel, bay or river, its front slope be- 
comes short and steep, and its rear slope becomes long and less 
inclined. Hence arises the fact that at a station near the sea the 
time occupied by the rise is equal to that occupied by the 
descent; but at a station more removed from the sea the rise 
occupies a shorter time than the descent. Thus, in Delaware 
bay and river, we have the following relations of the duration 
and height of rise and fall. 
STATION, Mean rise| Luni-tida | MEAN DURATION OF 
|_| Satall: | interval: lod tide. | Bbb tide. 
Feet. h. m. hm h. m. 
had held 3°5 8: 0 6 3 
= al Bias 6 4 5°56 6°30 
rm + 6S 11°53 5°24 ™ 2 
orseuereraag 6° 13°44 4-52 134 
