NATURAL PHILOSOPHY. 115 



seas, there is an efflux of air both to the north and the south. From 

 the south side of this zone the air flows in a never-ceasing breeze, called 

 the north-east trade winds, towards the equator. On the north side the 

 prevailing Avinds come from it also ; but they go towards the north- 

 east. They are the well known south-westerly winds which prevail along 

 the route from this to England. Both these winds are surface breezes. 

 From the equatorial calms there is a perpetual upper current to the 

 tropical calms, equal in volume to the trade winds. One peculiarity 

 of the trade winds is, that the south-west breezes give out a great 

 deal of moisture, proceeding from a calm belt to cooler regions, in a 

 course where precipitation is the natural result. The north-east trade 

 winds, on the other hand, proceeding from the same belt of calms, are 

 dry at the very outset. It might be supposed that the upper current, 

 which flowed from the calms at the equator, descended at the cahn 

 belt at the tropics, and then returned on the surface as a trade wind, 

 then ascended at the equator, returning as an upper current, thus keep- 

 ing up a continual ring of breezes. Lieut. Maury says, he knew of no 

 agent in nature that would prevent the winds taking this circuit ; but, 

 on the other hand, he knew of circumstances which rendered it prob- 

 able that such in general is not the course of atmospheric circulation. 

 But there are also south-east trade winds ; and Lieut. Maury has come to 

 the conclusion, that the current which flows to the equator as a surface 

 north-east trade wind, ascends at the equatorial calms, and passes to 

 the south as an upper current, while the current which comes as the 

 south-east trade winds ascended and passed to the cahn zone of Cancer. 

 The reasons for this conclusion are, that the evaporating surface of the 

 south is the greatest, but all the great rivers are in the northern hemi- 

 sphere, and at those seasons of the year, when the sun is evaporating 

 most at the south, the greatest quantity of rain is falling in the north- 

 ern hemisphere. Without taking this view of the subject, Lieut. Maury 

 " could find no part of the ocean of the northern hemisphere from which 

 the sources of the great rivers, Mississippi, St. Lawrence, and others, 

 could be supplied. It appeared to me," he says, " that the extra trop- 

 ical regions of the northern hemisphere stood in the relation of a con- 

 denser to a grand steam-machine, the boiler of which was in the region 

 of the south-east trade winds," and the north-west trade winds to the 

 tropic of Capricorn on the other side of the equator, perform the samo 

 office to the regions beyond that tropic, which the south-east winds 

 perform for our northern regions. Meteorological observations, made in 

 various parts of the south-western states, show, in confirmation of these 

 views, that the south-west winds are generally the rain-bearing winds ; 

 but a further, and almost conclusive proof is found in the fact that 

 Ehrenburg has detected in the blood rains of the South of Europe, 

 infusoria from South America. It is, therefore, probable that the trade- 

 winds of the southern hemisphere, after arriving at the belt of equa- 

 torial calms, ascend and continue in their course towards the calms of 

 Cancer as an upper current from the south-west, and after passing this 

 zone of calms, they are felt on the surface as the prevailing south-west 

 winds of the extra-tropical parts of our hemisphere ; and that for the 

 most part they bring their moisture with them from the trade wind 



