The mean tide of the two oceans is about the same.
| PACIFIC AT PANAMA. | PACIFIC AT PANAMA. | ATLANTIC AT ASPINWALL. | |
|---|---|---|---|
| MAY & JUNE | NOV. & DEC. | AUG. & SEPT. | |
| FEET. | FEET. | FEET. | |
| Greatest rise of tide | 17.72 | 21.30 | 1.60 |
| Least rise of tide | 7.94 | 9.70 | 0.62 |
| Average | 12.08 | 14.10 | 1.16 |
| Mean tide of Pacific above | |||
| mean tide of Atlantic | 0.759 | 0.14 | |
Mr. Lloyd found a difference of 27.44 feet between high and low water at Panama. The Red Sea is 3 inches higher than the Mediterranean. The Atlantic at Brest is 3½ feet higher than the Mediterranean at Marseilles.
The small variation in the mean tide at Panama of the two oceans is probably due to the action of winds and the Gulf Stream. At Panama the highest flood tide rises about ten and one-half feet above the level of the mean tide of the Atlantic, and the extreme ebb falls about the same number of feet below it. The alternate currents through the new strait, caused by the rise and fall of the tide, would prove a serious inconvenience to navigation.
The Pacific tide, piling up at the head of the new cut, and entering the strait with considerable violence, would be propelled toward the Gulf in a manner analogous to the progression of the tidal wave in a river. Upon the ebb of the tide a reverse current would prevail. Navigation would not only be obstructed by these alternate currents, but the channel would be choked by drifting timber washed into the canal during the rainy season. Silt and sand would be deposited in bars at the outlet of the canal, or swept inward to form shoals where the current could no longer transport it.
Mr. Gisborne, in his report, devotes some space to speculations on these results. “There can be no doubt,” he remarks, “that at high water there will be a current from the Pacific to the Atlantic, and that during the ebb tide there will be a current in the opposite direction. The extent of these currents, and the place of their greatest effect, depends on the comparative sectional area of different portions; and if the cross-section is uniform throughout, will be some time after high tide in the Pacific and at the Atlantic end of the canal. The phase of the tide wave (or the appreciable effect of the tide) will take one and one-half hours to reach from one end to the other, and presuming the current to be uniform in the whole length”——“the question may be examined as a maximum, i. e., what will be the surface velocity of a canal thirty miles long, having a fall of eleven feet, or with a horizontal bottom having at one end twenty-eight feet, and at the other thirty-nine?”
Employing Du Buat’s formula, with the following quantities:
| Mean depth | 35.50 | feet. |
| Mean width | 183.50 | “ |
| Mean border | 244.80 | “ |
| Area water section | 6,147.255 | “ |
| Hydraulic mean depth | 25.11 | “ |
| Fall per mile | 0.33 | “ |
he deduces a maximum surface velocity of three miles per hour. The assumed average fall per mile is strictly a variable function, and at its maximum would give a result greatly in excess of that deduced by Mr. Gisborne.
There is no reason for this assumption of a fall of 0.33 of a foot per mile. It directly involves the question to be determined, since the velocity depends upon the inclination of the surface. The value deduced by the formula is not the maximum but the minimum velocity attained in the canal upon the assumed fall per mile.