"Rear-Admiral FitzRoy on Meteorological Telegraphy. 397 



varieties of weather that are experienced as the wind veers more 

 toward or from the equator or the nearest pole ; and sometimes so 

 antagonistic in their angular collision as to cause those large circling 

 eddies or rotatory storms called cyclones (in modern parlance), which 

 are really like the greater storms in all parts of the world, although 

 they do not quite assimilate to those local whirlwinds, dust-storms, 

 and other commotions of atmosphere which seem to be more elec- 

 trical in their characteristics, if net in their origin. 



Whenever a polar current prevails at anyplace, 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, specifi- 

 cally lighter ; and these changes take place slowly. 



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

 nical — either current, or any combination of currents, ceases to press 

 onward without being opposed, a gradual lightening of the atmosphere 

 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 disposable 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 assi- 

 milated to the characteristics of those extremes ; while all the varia- 

 tions of pressure or tension, many of them 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 betwen 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 

 upward (only a few thousand yards, or even feet), and that high 

 land, mountains, especially ranges of mountains, alter and impede 

 its progress, a variety of eddy winds, or, as it w r ere, streams of wind, 

 with local and apparentlyanomalous effects, must be frequently caused. 



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

 fog causing (heat), or its other changes, namely, evaporation, rare- 

 faction, 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- 



