J. E. Hilgard—Tidal Waves and Currents, 121 
of the ebb tide, which depends upon the unchanged factors of 
area and form of basin, height of tide, and character of the 
material forming the bar. 
Charleston bar.—An interesting instance of this maintenance 
of the depth of channels from a determinate tidal basin is fur- 
nished by the effects of the obstructions placed in the channel 
over Charleston bar during the war of the rebellion. On the 
accompanying diagram is seen the “stone fleet” sunk in the 
main channel, which at that time had twelve feet of water at 
low tide where the figure 7 indicates the present depth. There 
was, moreover, another channel making out more to the south- 
ward, with nine feet of water where the figure 3 indicates the 
present depth. The vessels were placed checkerwise in such a 
manner as to impede navigation while interfering least with the 
discharge of the water. The effect, nevertheless, was the forma- 
tion of a shoal in a short time, and the scouring out of two 
channels, one on each side of the obstructions, through which 
twelve and fourteen feet can now be carried at low water. The 
increased water-way thus given to the ebb tide caused it to 
abandon the old nine foot channel on the less direct course to 
deep water. We have here the total obstruction of a channel, 
which was of considerable importance to the southward trade, 
by new conditions nan eh at a point four miles distant 
from where the effect was produced; and we are warned how 
hours after the transit of the moon high water has advanced 
just within Block Island with an elevation of two feet, and, at 
the same time, has passed Sandy Hook with an elevation of four 
and a half feet. Traversing the sound at a rate indicated by 
the Roman figures, with increasing heights indicated by the 
Arabic numerals, it reaches Sands’ Point eleven and a half 
hours after the transit of the moon with a height of seven and 
Seven-tenths feet. The observed time of transmission from the 
vace to Sands’ Point is two hours and one minute, and the 
time computed from the depths, according to the law developed 
by Airy, is two hours fourteen minutes; a very good approxi- 
mation when we consider the irregularities in the configuration 
of the Sound which could not be taken into account. Advane- 
