Barometric Anomalies about the Andes. 389 
___ to windward of the Andes, onght there not to be a hollow or de- 
pression in it to leeward of them also? With the ®iew of get- 
ting some light with regard to the answer to this question, I have 
Sought to ascertain what is the mean height of the barometer at 
Lima and along the Peruvian coast of South America. It appears 
that the mean height of the barometer at Lima is, according to 
Doctor Unanue, 29-13, (27 pulgadas, 4 lineas,) with a variation 
in its range of from 2 to 4 dineas (0i-18 to 0"-37). He says the 
barometer rises 2 /ineas in the summer, and falls as much in win- 
ter. Assuming the mean height of the barometer at the sea level 
in Callao to be 30 in., Unanue’s mean reading would give 765 feet 
as the height of Lima above the sea. But according to a level 
run for the railroad between the two places, the height of Lima 
above the sea level of Callao, is only 496 feet, and until it was 
thus run, the height of Lima above the sea was generally 
; —— at what the barometer would make it, viz.: about 750 
eet 
The change in the barometric pressure due a height of 496 
feet, is one-third of an inch, and this correction being applied to 
the Lima barometer of Unanue to reduce it to the sea level, 
_- Would make the mean reading of the barometer at Callao to be 
29-46; thus confirming this conjecture, (so far as these scanty 
t observations go,) that the barometric pressure along this part of the 
Coast, is less than that due to its latitude and elevation. Admitting 
these conjectures to be truths, we derive a practical rule, that the 
ight of a chain of mountains determined by barometric pres- 
depends upon the way the wind blows. 
_ If the standard for comparison be placed at the foot of the 
Mountain on the windwatft side, the height of the mountain will 
appear too great; and if it be placed on the lee side, the height 
of the mountain will be too low. 
Lima is far enough, or nearly far enough south, to be beyond 
the reach of the diminished barometric pressure due to the belt of 
quatorial calms. But Lima may be under the hollow or de- 
pression caused in the atmosphere by the Andes, or rather in con- 
Sequence of the obstruction which these mountains oppose to the 
trade winds. The effect of this obstruction, as before explained, 
18 to cause a banking up in the atmosphere on the windward side 
of the Andes, (as there was found to be over Nauta,) and a de- 
pression in the air on their lee side. Whether Lima is under the 
axis of this atmospherical valley or not, or whether it 1s on one 
. 
aera 
