572 WINDS OP THE GLOBE. 



very variable climate, and this is known to be the case in the United States. 

 Nowhere in the same latitudes are the variations of temperature and pressure so 

 great and sudden as in the Mississippi Valley and in Texas. On the Atlantic 

 sea-board the variation is somewhat less, owing to the slight protection afforded by 

 the Appalachian Chain. 



In summer again, there are no parts of North America which are as strongly 

 heated as the interior parts of Asia, none also where pressure is as low, and tlius 

 there are no monsoons comparable in strength and constancy to the summer mon- 

 soons of Asia. Especially is this the case with the eastern part of the United 

 States, where the land is so much pervaded by the influence of the sea that there 

 is scarcely a summer depression of the barometer. The Gulf of "Mexico is situated 

 just in the latitudes where pressure would be lowest on a great continent, and, 

 owing to the relative coolness of the air over great bodies of water, pressure is 

 nearly as high over the Gulf in summer as in Avinter. Yet, as there is a rarefac- 

 tion of the air in the interior and western part of North America, there is a mon- 

 soon wind drawn in from the Gulf of Mexico to supply the deficiency. The mean 

 direction of the wind is southerly in summer over a great part of the United 

 States east of the Rocky Mountains. It is more S. E. in Texas, and S. and even 

 S. W. in the States north and northeast of it, partly due to ♦he earth's rotation, and 

 partly also to the influence of the lower pressure in the lake regions on the air 

 over the Gulf of Mexico. On the Atlantic coast the winds have some monsoon 

 features (as was shown by Prof. Coffin in 1848) but still the flow of air is much 

 more from the southwest than would be the case in a real monsoon region, the 

 ocean being to the east. 



If, aside from disturbing influences, we consider only the mean direction of the 

 wind, the influence of the Gulf of Mexico is seen to be paramount over a large 

 and important region of the United iStates, extending from the Mississippi to the 

 Appalachian Chain and from 34° to 42° N. L. The mean direction of the wind 

 is about W. S. W. at all seasons, with a ratio of resultant of about 30. The cause 

 of this is, that pressure is highest at all seasons to the S. and lowest to the N. aird 

 N. E. 



Having now considered the influence of the pressure of the air on the direction 

 of the winds, the influence on force remains to be shown. 



It is easy to conceive, that, the influence of pressure once acknoAvledged, this 

 influence would be the greater, the nearer areas of high pressure approach areas 

 of low pressure, or, in other words, the nearer any given difference of pressure was 

 found to exist. It was to be supposed, that the more this was the case, the greater 

 would be the velocity of the winds. This has been found to be really the case. 



This difference of pressure relative to distance was called by Stevenson harome- 

 trio gradient. This term of barometric gradient may be applied to the mean 

 direction of the wind, and the rate of progress, as well as to any given single obser- 

 vation. The more the isobars are crowded together, the steeper is the gradient, 

 and the greater will be the velocity of the wind, all other conditions being the 

 same. 



There are conditions well known to science in a general way, although not 



