42 
ENGINEERING: H. L. ABBOT 
power to operate the canal and railroad, and for the electric lighting 
of the Canal Zone. It should not be forgotten, however, that if a 
larger volume of water be desirable for these and other uses, the plan 
proposed by the New French Company to supplement the volume of 
its smaller projected lake is still available; namely, the construction 
of a masonry dam near Alhajuela, where a good site exists for creating 
an upper lake to hold back the surplus water which during the rainy 
months now runs to waste through the spillway. 
The Canal Zone lies between the ninth and tenth degrees of north 
latitude in a region of exceptional rainfall; where the sun, closely fol- 
lowed by rain clouds raised from the oceans in his annual journey be- 
tween the tropics, exerts a controlling influence upon the volume of 
local rainfall. When near his southern limit, in January, February, 
March and April, precipitation upon the Isthmus is at its minimum; 
December and May are usually intermediate in volume; in the remain- 
ing six months, when near his northern limit, heavy downpours are the 
rule. Furthermore, the local annual rainfall is not uniform across the 
Isthmus. As one passes from the Atlantic to the Pacific coast, the 
volume falls off gradually from about 130 inches at Colon to about 70 
inches at Panama. It will be noticed that even the latter is more than 
double the usual downfall in the United States, a fortunate circumstance 
for our great artery of commerce. Another local advantage is the fact 
that the atmosphere of the Isthmus is nearly saturated with aqueous 
vapor, which largely reduces the losses by lake evaporation. Our 
hydraulic problem seems therefore to be specially concerned with 
Isthmian rainfall and outflow, and the relation between them. 
To determine accurately the average annual rainfall at any locality, 
the records should cover at least half a century. Although the Isthmus 
has been known to civilization for more than four hundred years, the 
first annual rainfall records date from 1863, when they were begun by 
the agents of the Panama Railroad Company. As to the outflow from 
the watershed, the records are even less complete. The volume re- 
ceived from the clouds, after reduction by evaporation, by plant growth, 
and by possible infiltration, represents the available flow at the dam 
site. To determine to what extent existing data throw light upon this 
quantity, has seemed to me to be of primary interest. 
The earlier records are given in a paper published in the Monthly 
Weather Review in May, 1899; they include those of the Railroad com- 
pany, and those at that date collected by the two French Companies 
and the Liquidation. A gap of a few years in the former is supplied 
by a paper by Mr. A. P. Davis, published in the Twenty-second Annual 
