THE ATSIOSPHEKE. 91 



amongst the particles of aii\ In violent winds especially are these 

 upward and downward ciuTents made obvious by the leathers, 

 leaves, thistledown, dust, and trash that are blown about. It 

 would be well if our wind gauges and vanes therefore were so 

 constructed as to show the inclination as well as the azimuth of the 

 wind. With such an improvement we might ascertain whether certain 

 sudden changes in the weather be not owing quite as much to the 

 inclination as to the direction of the wind. 



246. Y/e may seek in the vertical chculation of the atmosphere 

 The hot xMnds of for an explanation in part, not only of hot and cold 

 the Andes. tcrms, but iu a measm'e also of seasons of exces- 

 sive drought, as well as of other phenomena with which all are 

 familiar. Travellers in crossing the Andes tell of hot v/inds 

 encountered there even on the mountain tops. Streaks of hot air 

 are also frequently encountered in various parts of America, and I 

 have no doubt in other countries also. 



247. To explain one of these sudden and severe "hot spells," let 

 Certain "Hot Spells " i^s supposo tlio neighbouring atmosphcre to be weU 

 explained. loaded T\dth moisturo at the tem;^» era tore of 80^ for 

 example, and with the barometer at 30 in. ; that from some cause 

 this rain-laden air commences to ascend, and its vapour to be 

 condensed. In this process the heat which was latent in the 

 vapour becomes sensible in the air. Now the height to which this 

 air rises may be such, were it dry air, as to reduce its temperature 

 80^, and bring it down to zero ; but it is moist air, and the 

 hberated heat may be sufficient to raise it to 20°, and so 

 prevent the temperatm^e from going below that reading. Thus 

 this air is at least 20^ above* the normal temperature of the 

 height to which it may have risen. Suppose that now, in the 

 process of vertical circulation, it be brought down to the surface 

 again, and submitted to the same barometric pressure as before : its , 

 temperature now will not be 80', as before, but it will be 80^ -\- 20^, 

 or 100^. Thus by going up, precipitating its moisture ; and coming 

 down, it is made hot. 



248. Whenever and wherever air in this condition descends to the 

 Reservoirs in the surfacc, there will bc a longer or shorter period of 

 *^^" excessively warm weather. t Thus we infer the 



* Balloonists often in their voyages pass throngli layers of warm and cool air, 

 made so doubtless by unequal radiation on one liand, or the liberation of the 

 latent heat of vapour on the otlier. 



t " Heated Wind Storm. — A heated wind storm passed over a portion of 



