THE ATMOSPHERE. 95 



otlier evidence besides that of induction (§ 224) as to upward 

 and downward movements amongst tlie particles of air. In 

 violent winds especially are these upward and downward cur- 

 rents made obvious by the feathers, leaves, thistledown, dust, 

 and trash that are blown about. It would be well if our wind 

 £rau2:es 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 owdng quite as much to the inclination as to 

 the direction of the wind. 



246. TJie hot ivinds of the A7ides. — We may seek in the vertical 

 circulation of the atmosphere for an explanation in part, not only 

 of hot and cold terms, but in a measure 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 winds 

 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. Certain " Hot Spells " explained. — To explain one of these 

 sudden and severe " hot spells," let us suppose the neighbouring 

 atmosphere to be well loaded with moisture at the temperature 

 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 aii\ 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 liberated heat may be sufficient to raise it to 

 20^, and so prevent the temperature from going below that read- 

 ing. Thus this air is at least 20° above* the normal tempera- 

 ture 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. Reservoirs in the sky. — Whenever and wherever air in this 

 condition descends to the surface, there will be a longer or 



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

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

 latent heat of vapour on the othei-. 



