ENGINEERING EEPOETS. 
17 
the mile ; thus forming, as it were, an immense inclined plane, 
with its side next the mountains, about two hundred and fifty 
feet above the Pacific. Under these circumstances they pre- 
sent a remarkably smooth, even surface, with a uniform, gentle 
slope towards the sea. In some instances there are occasional 
isolated hills, which, rising abruptly, form a prominent feature 
in the topography of this part of the country. The plains are 
traversed by eight rivers, which discharge the drainage of the 
southern slope into the sea. Seven of these rivers empty into 
the lagoons, which are connected with the sea by a narrow out- 
let, called the Boca Barra : the eighth, or Tehuantepec River, 
comes from a northwesterly direction, and passing through the 
city of the same name, discharges itself directly into the sea at 
the Bay of Yentosa. 
The most important of the streams referred to, as respects 
length and the volume of their waters, are the Ostuta and Chica- 
pa on the east, and the Tehuantepec on the west. The first two 
named rivers have their source in the highest parts of the Sierra 
to the east of San Miguel Chimalapa. It is said of them that 
they always rise and fall simultaneously, the slightest change 
in one stream being accompanied by a corresponding variation 
in the other — a fact which has originated the belief that they 
have a common source in a lake supposed to be on the summit 
of the mountains to the east ; though the true reasoifis no doubt 
to be found in the fact that they botls^roceed from the high- 
est points of the Sierra, and through the upper part of their 
course are in close proximity to each other. These streams de- 
rive their chief importance as being the source from which a 
sufficient supply of water may probably be obtained for the 
summit-level of a ship-canal. The plan proposed by Mr. Moro 
for obtaining this supply was, " to open a trench that would lead 
the waters of the Ostuta to the valley of the Chicapa ; and an- 
other, which, beginning a little above the 4 Ultimo Rancho,' 
would direct the waters thus collected to the table-lands of Ta- 
rifa, following the brows of the hills to the north of the valley 
watered by the upper Chicapa and the Monetza." The average 
depth of water in the Chicapa (at San Miguel Chimalapa), from 
June to October inclusive, is seven feet, and a section of the 
b 
