SKCT. XV. ACTION OP THE SUN AND MOON. 115 



tropics or in the higher latitudes, in consequence of the 

 ascent of the heated air from the surface of the earth ; 

 it is less also on the shores of the Baltic sea than it is 

 in France, probably from some permanent eddy in the 

 air arising from the conformation of the surrounding 

 land ; and to similar local causes those barometric depres- 

 sions may be attributed which have been observed by 

 M. Erman, near the Sea of Ochotzk in Eastern Siberia, 

 and by Captain Foster near Cape Horn. 



There are various periodic oscillations in the atmos- 

 phere which, rising and falling like waves in the sea, 

 occasion corresponding changes in the height of the 

 barometer, but they differ as much from the trade winds, 

 monsoons, and other currents, as the tides of the sea do 

 from the Gulf-stream and other oceanic rivers. The 

 sun and moon disturb the equilibrium of the atmosphere 

 by their attraction, and produce annual undulations which 

 have their maximum altitudes at the equinoxes and their 

 minima at the solstices. There are also lunar tides 

 which ebb and flow twice in the course of a lunation. 

 The diurnal tides, which accomplish their rise and fall 

 in six hours, are greatly modified by the heat of the 

 sun. Between the tropics the barometer attains its 

 maximum height about nine hi the morning, then sinks 

 till three or four in the afternoon; it again rises and 

 attains a second maximum about nine in the evening, 

 and then it begins to fall and reaches a second minimum 

 at three in the morning, again to pursue the same course. 

 According to M. Bouvard, the amount of the oscillations 

 at the equator is proportional to the temperature, and 

 in other parallels it varies as the temperature and the 

 square o'f the cosine of the latitude conjointly, conse- 

 quently it decreases from the equator to the poles, but 

 it is somewhat greater in the day than in the night. 



Besides these small undulations, there are vast waves 

 perpetually moving over the continents and oceans in 

 separate and independent systems, being confined to 

 local yet very extensive districts, probably occasioned by 

 long-continued rains or dry weather over large tracts of 

 country. By numerous barometrical observations made 

 simultaneously in both hemispheres, the courses of sev- 

 eral have been traced, some of which occupy twenty -four 



