INTENSITY OY LIGHT. 
351 
tlie last, which perhaps had never before been employed, 
might he rendered nearly exact, by adding a scale ol equal 
parts to the moveable frame of the telescope of the sextant. 
1 1 was by taking the mean of a great number of valuations, 
that I saw the relative intensity of the light of the great 
stars decrease in the following manner: Sirius, Canopus, 
a Centauri, Acherner, (3 Centauri, Fomalhaut, Bigel, Pro- 
cyon, Beteigueuse, t of the Great Dog, 5 ot the Great 
Dog, a of the Crane, a of the Peacock. These experiments 
will become more interesting when travellers shall have 
determined anew, at intervals of forty or fifty years, some 
of those changes which the celestial bodies seem to undergo, 
either at their surface or with respect to their distances from 
our planetary system. 
After having made astronomical observations with the 
same instruments, in our northern climates and in the 
torrid zone, we are surprised at the effect produced in the 
latter (by the transparency of the air, and the less extinc- 
tion of light), on the clearness with which the double stars, 
the satellites of Jupiter, or certain nebula), present them- 
selves. Beneath a sky equally serene in appearance, it 
would seem as if more perfect instruments were employed ; 
so much more distinct and well defined do the objects 
appear between the tropics. It cannot be doubted, that 
at tlic period when equinoctial America shall become the 
centre of extensive civilization, physical _ astronomy will 
make immense improvements, in proportion as the skies 
will he explored with excellent glasses, in the dry and liot 
climates of Cumana, Coro, and the island of Margarets. 
I do not here mention the ridge of the Cordilleras, because, 
with the exception of some high and nearly barren plains 
in Mexico and Peru, the very elevated table-lands, in which 
the barometric pressure is from ten to twelve inches less 
than at the level of the sea, have a misty and extremely 
variable climate. The extreme purity of the atmosphere 
which constantly prevails in the low regions during the 
dry season, counterbalances the elevation of site and the 
rarity of the air on the table-lands. The elevated strata 
of the atmosphere, when they envelope the ridges of moun- 
tains, undergo rapid changes in their transparency. 
The night of the 11th of November was cool and ex- 
