SECT, xv.] ACTION OF THE SUN AND MOON. 135 



other oceanic rivers. The sun and moon disturb the equi- 

 librium 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 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 con- 

 jointly ; 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 barometrical 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 ascertained. Although, like all 

 other waves, these are but moving forms, yet winds arise 

 dependent on them like tide streams in the ocean. Mr. Birt 

 has determined the periods of other waves of still greater 

 extent and duration, two of which require seventeen days 

 to rise and fall, and another took thirteen days to complete 



