Ihe Tides. 209 



1852 the tide at Fort Delaware fell continuously for 36 hours in 

 ■consequence of the wind blowing down stream, and instead of six 

 feet the fall was actually fourteen. 



The tides have their origin in the oceans and thence proceed to 

 our shores, part of the time as forced and part of the time as free 

 waves, that is to say, they are moving part of the time under the 

 action of the tide-making force and when by the earth's rotation 

 they are removed from this influence they continue under their 

 own vis-viva until again brought within the influence of the same 

 cause. After investigating some of the peculiarities of the tides 

 on the coasts, we will return to the ocean wave. After reaching 

 the coast the tide enters every bay and river within its scope, 

 and, while doing so, undergoes many modifications. The range 

 is subject to change with every change in the cross-section, so that 

 observations along a river will vary considerable even at short 

 distances. In extensive bays the fact is more marked than in 

 rivers, the range, which is increased by the contraction of the 

 inlet, is at once diminished when the wave enters and spreads in 

 the basin, but, while the influx is retarded, the main wave passes 

 by, the ocean falls and efilax begins before the bay is filled to the 

 •ocean level, so that the bay never rises to a level with the ocean 

 and for the same reason also, never becomes as low. The Dela- 

 ware and Chesapsake bays are cases of this kind; the Mexican 

 gulf is one of the most extreme and will receive special notice 

 further on. 



The time card of steamers carrying on small tidal rivers is a 

 curiosity to those not familiar with such rivers, some of which 

 have scarce one foot of water at their entrance at low tide, so that 

 the boats are obliged to enter and leave the river on the tide wave 

 and their time must vary from day to day as we find the tides 

 do var}'. On large tidal rivers the case is different. Vessels will 

 meet several tides during one trip, as, for instance, on the Hudson 

 river, New York. The tide which passes New York city at 

 8:13 A. AL, reaches Albany at 3:30 P. M., with a mean velocity 

 of about 17 miles an hour so that the length of the half wave, 

 from high to low water, is about 100 miles. 



A boat leaving Albany at high water, at say 15 miles an hour, 

 14 



