10 1 THE POPULAR SCIENCE MONTHLY. 



is five thousand feet above the plateau. (Appletons' Physical Geog- 

 raphy, p. 23). The latter furnish the second impediment to the 

 onward progress of these winds, since they run southeast-north- 

 west ; but the trades blow directly west, and thus impinge upon 

 them at an angle which deflects the winds themselves toward 

 the north. 



The Sierra Madre are more than one thousand miles long, and 

 are an insuperable barrier to the progress of these vapor-loaded 

 winds. This is evident, as there is no indication of their pres- 

 ence on the west side, neither on the land nor on the water, as the 

 Pacific trade winds appear to originate about one hundred and 

 fifty miles west of the coast of Mexico. An analogous case is 

 cited by Prof. Orton, in his Andes and the Amazon, p. 118, who 

 says, when speaking of the Andes, " So effective is that barrier 

 that the trade winds are not felt again on the Pacific till you are 

 one hundred and fifty miles from the coast." 



These winds appear to be shoved up, strata upon strata, on the 

 Mexican plateau, and when they finally reach the Sierra Madre 

 Mountains, over which they can not -pass, they are rolled back 

 upon themselves. They must have an outlet. The rushing wind 

 from the east prevents their moving in that direction, and the 

 force of the main current forbids their flowing toward the equa- 

 tor, and thus their outlet can only be toward the north. They 

 are now so high that they must be beyond the influence of the 

 rotary motion of the earth, and are governed by the force of 

 gravitation alone. In accordance with the latter law they flow, 

 as on an inclined plane, over the colder and more dense air toward 

 the north, and thus restore the equilibrium of the atmosphere 

 that has been disturbed. This disturbance is caused by a con- 

 tinual flow of the cold and heavy surface air from the extreme 

 north toward the equator, because along the tropical belt a par- 

 tial vacuum is created by the air becoming heated and lighter 

 and in consequence floating upward, and the cold air rushes in to 

 supply that vacuum. 



These comparatively warm strata, though high in the atmos- 

 phere, have a tendency to reach the earth, but, being lighter than 

 the surface air, they float above it until their respective densities 

 are about the same. The point of contact with the earth of the 

 lower strata of these " return trades " is near 30 north latitude in 

 the summer, but still further north in the winter. This point of 

 contact is near and along the north shore of the gulf, and the 

 blending of the moisture of the " return trades " with that off the 

 gulf may account for the unusually large rainfall of sixty inches 

 near that line ; meanwhile the main and higher strata blow on 

 and reach the earth further north. 



" The polar winds, seeking the equator, strike obliquely against 



