266 POPULAR SCIENCE MONTHLY. 



rtt six inches in depth per month from the surface of the lake will 

 amount to about 1,070 cu. ft. per second if the traffic through the canal 

 should amount to 10,000,000 tons per annum in ships of ordinary size. 

 This draft per second is the sum of 406 cu. ft. per second for lockage, 

 207 for evaporation, 250 for leakage at the lock gates and 200 for power 

 and other purposes, making a total of 1,063, which has been taken as 

 1,070 cu. ft. per second. The amount of storage in Lake Bohio between 

 the elevations of 85 and 82 ft. above sea-level, as designed, is sufficient 

 to supply the needs of that traffic in excess of the smallest recorded low- 

 water flow of the Chagres Eiver during the dry season of a low rainfall 

 year. The lowest monthly average flow of the Chagres on record at 

 Bohio is 600 cu. ft. per second for March, 1891, and for the purposes 

 of this computation that minimum flow has been supposed to con- 

 tinue for three months. This includes a sensible margin of safety. 

 In not even the driest year therefore can it be reasonably expected that 

 the summit level of the canal would fall below the elevation of 82 ft. 

 until the total traffic of the canal carried in ships of the present ordinary 

 size shall exceed 10,000,000 tons. If the average size of ships continues 

 to increase, as will probably be the case, less water in proportion to ton- 

 nage will be required for the purposes of lockage. This follows from 

 the fact that with a given tonnage the greater the capacity of the ships 

 the less the number required, and consequently the less will be the 

 number of lockages made. 



On the other hand it can be shown that with a depth of five feet of 

 water on the crest of the Gigante Spillway the discharge of that weir 

 2,000 ft. long will be at the rate of 78,260 cu. ft. per second. If the 

 flood water of the Chagres should flow into Lake Bohio until the head 

 of water on the crest of the Gigante weir rises to 7.5 ft. the rate of 

 discharge over that weir would be 140,000 cu. ft. per second, which, as 

 already shown, exceeds at least by a little the highest flood rate on 

 record. The operation of Lake Bohio as a flood controller or regulator 

 is therefore exceedingly simple. The flood waters of the Chagres would 

 pour into the lake and immediately begin to flow over the Gigante 

 weir, and continue to do so at an increasing rate as the flood continues. 

 The discharge of the weir is augmented by the increasing flood and 

 decreases only after the passage of the crest of the flood wave. No 

 flood even as great as the greatest sr.pposable flood on record can 

 increase the elevation of the lake more than 92 to 92.5 ft. above sea- 

 level, and it will only be at long intervals of time that floods will raise 

 that elevation more than about 90 ft. above sea level. The control is 

 automatic and unfailingly certain. It prevents absolutely any damage 

 from the highest supposable floods of the Chagres, and reserves in 

 Lake Bohio all that is required for the purposes of the canal and for 

 wastage by evaporation through the lowest rainfall season. The floods 



