SECT. XV. TRADE WINDS. 121 



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 tidesj 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 in 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 of the cosine of the latitude 

 conjointly ; consequently 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 per- 

 petually 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 barome- 

 trical observations made simultaneously in both hemispheres, the 

 courses of several have been traced, some of which occupy 

 twenty-four, and others thirty-six, hours to accomplish their 

 rise and fall. One especially of these vast barometric waves, 

 many hundreds of miles in breadth, has been traced over the 

 greater part of Europe ; and not its breadth only, but also the 

 direction of its front and its velocity, have been clearly ascer- 

 tained. Although, like all other waves, these are but moving 

 forms, yet winds arise dependent on then! like tide streams in 

 the ocean. Mr. Birt has determined the periods of other waves 

 of still greater extent and duration, two of which required 

 seventeen days to rise and fall ; and another which takes fourteen 

 days to complete its undulation, called by Mr. Birt the November 

 wave, passes annually over the British Islands, probably over 

 the whole of Europe and the seas on its northern coasts. Its 

 crest, which appears to be 6000 miles in extent, moves from 

 N.W. to S.E. at the rate of about 19 miles an hour ; while the 

 extent of its barometrical elevation from its trough to its crest is 



