280 Lieut. Maury on the Probable Relation between 



afforded by nature, if not in favour, certainly not against the 

 conjecture that such had been the voyage of this vapour 

 through the air. At any rate here was proof of the immense 

 quantity of vapour which these winds of the extra-tropical 

 regions carry along with them towards the poles, and I 

 could imagine no other place than that suggested, whence 

 these winds could get so much vapour. 



I am not unaware of the theory or of the weight attached 

 to it, which requires precipitation to take place in the upper 

 regions of the atmosphere on account of the cold there 

 irrespective of proximity to mountain tops and snow-clad 

 hills. 



But the facts and conditions developed by these charts are 

 in many respects irreconcilable with that theory. With a 

 new system of facts before me, I have, independent of all 

 preconceived notions and opinions, set about to seek among 

 them for explanations and reconciliations. 



Arrived at this stage in the process of deduction and fiad- 

 ing conformity, the next step was to trace back the vapour 

 that supplies the sources of the Mississippi river and its 

 tributaries with rains, to its place in the ocean whence it 

 came. 



It rains more in the valley drained by that river than is 

 evaporated from it again. The difference for a year is the 

 volume of water annually discharged by that river into the 

 sea. 



At the time and place that the vapour which supplies this 

 immense volume of water was lifted by the atmosphere up 

 from the sea, it was reasoned that the thermometer stood 

 higher than it did at the time and place where it was con- 

 densed and fell down as rain in the Mississippi valley. 



I looked to the south for the springs in the sea which 

 supply the fountains of this river with rain. But I could not 

 find spare evaporating surface enough for it in the first 

 place ; and if the vapour, I could not find the winds which 

 would convey it to the right place. 



The prevailing winds in the Caribbean Sea and southern 

 parts of the Gulf of Mexico are the NE. trade winds. 

 They have their offices to perform in the river basins of 



