446 Admiral Fitz-Roy [March 28, 



of atmosphere which seem to be more electrical in their characteristics, 

 if not in their origin. 



Whenever a polar current prevails at any place, or is approaching^ 

 the air becomes heavy, and the barometer is high or rises. When the 

 opposite (equatorial or tropical) prevails or approaches, the mercury is 

 low or falls, because the air is, or is becoming, specifically lighter, and 

 these changes take place slowly. 



Whenever, from any causes — electrical, chemical, or simply 

 mechanical — either current, or any combination of currents, ceases to 

 press onward without being opposed, a gradual lightening of the atmo- 

 sphere, through a greater or less area of hundreds, or perhaps thousands 

 of miles occurs, not suddenly, but very gradually, and the barometer 

 falls. There is less tension. 



To restore equilibrium, the nearest deposable body of air (so to 

 speak) or most moveable, advances first ; but an impulse, at the same 

 time, may be given to other and greater masses that — though later in 

 arriving — may be stronger — last longer, and cause greater pressure 

 mechanically as well as by combination. Air, like water, mingles 

 very slowly, either from above or laterally. 



Taking, with Dove, north-east and south-west {true) as the " wind- 

 poles," all intermediate directions are found to be more or less assimi- 

 lated to the characteristics of those extremes ; while all the variations 

 of pressure, or tension, many of those caused by temperature, and all 

 varieties of winds, may be clearly and directly traced to the operations 

 of two constant principal currents — equatorial or tropical, and polar — 

 our north-east and south-west. 



Great distinction should be marked between those ever alternate 

 and often conflicting main currents — tropical and polar, and the local 

 effects of their union, or antagonism, namely mixed winds — whether 

 westerly or easterly, with occasional cyclones or circulating eddies, on 

 a large or small scale. 



Considering that the lower current does not ordinarily extend far up- 

 ward (only a few thousand yards, or even feet), and that high land, moun- 

 tains, especially ranges of mountains, alter and impede its progress, a 

 variety of eddy winds, or as it were streams of wind, with local and 

 apparently anomalous eflfects, must be frequently caused. 



Electrical action, condensation of vapour into hail, snow, rain, or 

 fog, causing heat; or its other changes, namely, evaporation, rarefac- 

 tion, and expansion — absorbing heat, and therefore causing cold — 

 immediately affect currents of air in a degree proportional to such 

 influence ; inducing horizontal motion. 



The polar current always advances from the polar quarter, while 

 laterally moving eastward (like a ship making lee-way), being pressed 

 towards the east by the tropical flow which advances from the south- 

 westward, usually above and at an angle with the polar stream or 

 current of air, often mixing with it, but at times separately penetrating 

 downward, then sweeping and warming the earth's surface, uncom- 

 bined with the polar current, even while feeling its approaching 



