745 



rary oscillations extremely uniform. Atthesame 

 hours the barometric heights scarcely differ 

 from 2 to 2i millimeters during whole months. 

 But sometimes the north winds, which are so 

 impetuous in the gulph of Mexico, blow back 

 the air as far as the table land of Anahuac, 

 and suddenly raise the mercury. This blowing 

 back caused the barometer to mount * on the 

 23rd March, 1783, at 10 h at night, 264 lines, 

 while the minimum of all the heights observed 

 in the whole year (20th January, 1783) was 

 259.3ii. In deducting the effect of the periodi- 

 cal oscillations, the extreme variations attain 

 at the same hour of the maxima and the minima, 

 by a concurrence of accidental circumstances, 

 at most 3.8 n or Si millimeters. We are sur- 

 prised to see this constancy in the extent of the 

 oscillations on a table land, where, in 19° 25' of 

 latitude, the thermometer descends in winter, 

 between 4 h and 5 h in the morning, several 



Dissertation fisica sobre la aurora boreal del 14 Nov. 1789, 

 p. 14). I saw the barometer of Mexico descend at the 

 epocha of the minimum, to 258.2 a (therm. 22° cent.) I saw 

 it highest at the hour of the maximum, 260 11 . (therm. 

 18.8°). 



* Political Essay, Vol. i, p. 83. During this blowing 

 back of the air, which is borne towards the boreal regions 

 of the south, the centigrade thermometer does not sink at 

 Vera- Cruz (in the lower layers of the atmosphere) lower 

 than from 20° to 18°, and at most to 16.5°. 



