492 
ZODIACAL LIGHT. 
eighteenth century mariners were little interested by any* 
thing not having immediate relation to the course of a ship, 
and the demands of navigation. 
However brilliant the zodiacal light in the dry valley of 
Tuy, I have observed it more beautiful still at the back of 
the Cordilleras of Mexico, on the banks of the lake of 
Tescuco, eleven hundred and sixty toises above the surface 
of the ocean. In the month of Jan nan', 1804, the light 
rose sometimes to more than G0° above the horizon. The 
Milky Way appeared to grow pale compared with the bril- 
liancy of the zodiacal light; and if small, bluish, scattered 
clouds were accumulated toward the west, it seemed as if 
the moon were about to rise. 
I must here relate another very singular fact. On the 
18th of January, and tho 15th of February, 1800, the 
intensity of the zodiacal light changed in a very perceptible 
manner, at intervals of two or three minutes. Sometimes 
it was very faint, at others it surpassed the brilliancy of the 
Milky Way in Sagittarius. The changes took place in the 
whole pyramid, especially toward the interior, tar from the 
edges. During these variations of the zodiacal light, the 
hygrometer indicated considerable dryness. The stars of 
the fourth and fifth magnitude appeared constantly to the 
naked eye with tho same degree of light. Ho stream of 
vapour was visible: nothing seemed to alter the transparency 
of the atmosphere. In other years I saw the zodiacal light 
augment in the southern hemisphere half an hour before 
its disappearance. Cassini admitted “that the zodiacal 
light was feebler in certain years, and then returned to its 
former brilliancy.” He thought that these slow changes 
were connected with “ the samo emanations which render 
the appearance of spots and faculse periodical on the solar 
disk.” But this excellent observer does not mention those 
changes of intensity in the zodiacal light which I have 
several times remarked within the tropics, in the space of 
a few minutes. Mairan asserts, that in France it is common 
enough to see tho zodiacal light, in the months of February 
and March, mingling with a kind of Aurora Borealis, which 
he calls ‘ undecided,’ and the nebulous matter of which 
spreads itself all around the horizon, or appears toward the 
west. I very much doubt, whether, in the observations I 
