298 M. Humboldt on the Horaiy 



The extent of the oscillations decreases, in proportion as the 

 latitude, and the annual deviations due to accidental pertur- 

 bations, increase. In the maxima of the evening, the mer- 

 cury is generally a little lower than in the maxima of the 

 mornino-. If we confine ourselves to observations that are 

 precise, and sufficiently numerous to give means worthy of 

 confidence, we find, that the extent of the oscillations under 

 the torrid zone, between the equator and the parallel of 10°, 

 in the tide of 9 h a. m. to 4 h p. m., is, in the plains, from 2.6 mm 

 to 3.3 mm ; on the plain of Bogota 1365 toises high, 2.3 mm ; 

 towards the extremity of the southern torrid zone, in the 

 plains 2 millimetres. In the whole year, the diurnal varia- 

 tions vary at Bogota from 0.63 mm to 3.64 mm , and the means 

 of the monthly oscillations vary from 1.5 mm to 2.7 mm . The 

 extent of the oscillations in the morning tides from 9 h to 4 b , 

 and in the evening from 4 h to ll h , are generally, under the 

 tropics, in the ratio of 5 : 4 or 5:3. The mean barometric 

 heights of the days vary between 0° and 10° of latitude, in the 

 plains, 3. 8 mm , and on the plain of Bogota 3 millimetres ; a dif- 

 ference of level, therefore, of 1G00 toises, has a very slight 

 influence on the mean of the daily oscillations, and on the ex- 

 tremes of these oscillations. The mean heights at noon, be- 

 tween the tropics, are constantly some tenths of a millimetre 

 hio-her than the general mean of the day, deduced from the 

 maximum of 9 h a. m., and the minimum of 4 h p. M. In ad- 

 vancing from the equator to the poles, we find the differences 

 of the barometrical heights at 9 h a. m. and 4 h p. m., between 

 0° and 20° of lat., from 2.5 mm to 3.0 mm ; between 28° and 30° 

 of lat. to 1.5 mra ; in 43° and 45° of lat. to 1.0 mm ; in 48°, 49° 

 0.8 mra , and in 55° of lat. to 0.2 rara . 



8. The barometrical means of the months differ from each 

 other under the tropics from 1.2 rom to 1.5 ram ; at Havannah, 

 Macao, and Rio Janeiro, near the tropics of Cancer and Ca- 

 pricorn, from 7 to 8 millimetres, as in the temperate zone. 

 The extreme deviations of the year are, at the same hours, near 

 the equator, from 4 to 4| millimetres. They rise sometimes at 

 the extremity of the equinoctial zone, near the tropic of Ca- 

 pricorn, to 21 mm , and near the tropic of Cancer to 25 and 30 

 millimetres. In the temperate regions of Europe, the limits 



