733 



two kinds of movements, which modify, and add 

 to each other ; and it may be said that the ba- 

 rometer remains lower one week than another 

 in the torrid, as in the temperate zone. The 

 knowledge of the limits of the absolute maxima 

 and minima is very important for the measure- 

 ment of heights by means of the barometer, 

 whenever between the tropics, without corres- 

 ponding observations, we deduce from a small 

 number of observations made on a particular 

 spot, and at certain hours of the day, the state 

 of the barometer at every consecutive hour of 

 day and night. Bouguer, La Condamine,, and the 

 greater part of the travellers who have spoken 

 after them of horary variations., confound the 

 extent of the oscillations corresponding to one 

 atmospheric tide, with the changes of the mean 

 heights of the barometer in different weeks or 

 different months. Bouguer says * that the co- 

 lumn of mercury in the torrid zone varies from 

 2f to 3 lines ; but that the variations at Quito 

 are only 1 line. The former part of this asser- 

 tion can relate only to the extreme accidental 



* Figure de la terre, p. 39. Caldas, in the Semanario, 

 Vol. i, p. 248. Don George Juan thought he remarked a 

 diminution in the extent of the oscillations, in proportion as 

 he approached from the tropic to the equator (Observ. Astro- 

 nomical, p. 99), He fixes this extent at Petit Goave, at 2} 

 line, and at Guayaquil, at \ \ line, 



