May 28, I860.] METEOROLOGICAL DEPARTMENT. 165 



winds or monsoons), and other parts divide, mix with, or oppose 

 the polar currents in a variety of ways, between the tropics and 

 arctic (or antarctic) regions. 



" Such currents sometimes flow side by side, though in opposite 

 directions, as ' parallel streams,' for hundreds or even thousands of 

 miles.* Sometimes they are more or less superposed — perhaps, or 

 indeed fi^equently crossing at various angles ;f sometimes combining, 

 and by the composition of their forces and qualities causing those 

 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 modem par- 

 lance), which are really like the greater storms in all parts of the 

 world, although they do not quite assimilate to those local whirl- 

 winds, dust-storms, and other commotions of atmosphere which are 

 more electrical in their origin and characteristics. 



" 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 (tropical or equatorial) prevails or approaches, the 

 mercury is low or falls, because the air is, or is becoming, specifi- 

 cally lighter, and these changes take place sIowIt/. 



*' Whenever, from any cause — electrical, chemical, or simply 

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

 to press onwards J 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, § 



*' To restore equilibrium, the nearest disposable body of air (so to 

 speak) moves first ; but an impulse, at the first time, may have 

 been 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 

 but slowly, either from above or laterally. 



* Like Sabine's currents of the sea, on the coast of Africa.—' Pendulum 

 Experiments.* 



t Green, Rush, and Welsh. — * Balloon Ascents.' 



J If opposed mechanical pressure increases ; and this may be caused by high 

 land, as well as by opposing wind. 



§ Evaporation, rarefaction, or condensation of vapour in air, reduces its specific 

 gravity — the two former by expanding bulk, and rendering it lighter ; the latter, 

 through mechanical diminution of quantity, by falling to the earth as rain, &c. 

 Moreover, there is more or less motion, avxiy from the place of stationary air, 

 which tends to lessen its elasticity or pressure, and cause the barometer to fall. 



