378 NOTES OF A NATURALIST. 



surface, have hitherto failed to lead to any general results 

 indicating a normal rate of decrease of temperature with in- 

 creasing elevation, it could scarcely be hoped that observations 

 on mountains should contribute farther to enlighten us. From 

 what has been already said, it is apparent that the fact that the 

 place of observation is close to the surface causes disturbances 

 the nature and amount of which must vary with each particular 

 spot, and with the season and the condition of the atmosphere 

 at the moment of observation. 



The intensity of solar radiation increases rapidly with in- 

 creasing elevation,* so that when the sky is clear surfaces 

 exposed to the sun are heated much above the normal tempera- 

 ture. Owing to its slight absorptive power the free atmosphere 

 is little affected ; but the strata nearest the surface are heated 

 by convection, while a contrary effect follows when the surface 

 is no longer exposed to the sun, and radiates freely to the sky. 



The air in mountain countries is rarely at rest. Even when 

 there is no sensible breeze, the unequal heating of the surface 

 causes ascending and descending currents, which lose or gain 

 heat by expansion or contraction. More commonly winds are 

 experienced which, by impinging on the inclined surfaces, force 

 bodies of air to higher elevations, and thereby directly cause a 

 fall of temperature. 



All these causes of disturbance are complicated by the action 

 of aqueous vapour, which, in most mountain countries, is sup- ' 

 plied from the surface, as well as borne upwards by ascending 

 currents. Besides the effect of raising the temperature where 

 condensation takes place, and lowering it where clouds are 

 dissolved in strata of dry air, the amount of aqueous vapour 

 present at a given place affects the intensity of solar radiation, 

 and the consequent amount of heating of the surface. 



In spite of these obstacles to the attainment of accurate 

 numerical results from which to infer the distribution of 

 temperature in the atmosphere, we are yet, for the larger part 

 of the earth, forced to rely on mountain observations as the only 



* On this subject see Handbuch der Klimatologie, by Julius Hann, 

 pp. 141, et seq. See also Tables I. and II. in a report on thermo- 

 metric observations in the Alps, by J. Ball, in Reports of the British 

 Association for the Advancement of Science for 1862, pp. 366-368. 



