CriMATE. 65 



seen that the photometer at 10 a. m., in the dry season, 

 frequently indicates 120° (12° oentissimal), when the ther- 

 mometer, in the shade, only shews a temperature of 60° or 

 61° (15'5 or 16'1), or to render the difference more intelK- 

 gible, when the temperature of the air in the shade is only 

 60°, the increase of heat from the sun's direct rays would 

 raise it to 81°6.* 



The effect of this increased power of the sun's rays on the 

 general temperature, and on the soil of elevated regions, 

 would be very considerable, were it not for certain disturbing 

 causes, the most powerful of which is the wind, which by 

 agitation of the atmosphere scatters the heat before it can 

 accumulate. When moving along the ground at the rate 

 of eight miles an hour, it diminishes the calorific action of 

 the light from the sun and sky one-half, but if it sweeps 

 with a velocity of 16, 24, or 32 miles in the hour, it will 

 reduce the whole effect successively to the 3rd, the 4th, or 

 the 5th of its standard.f The impression made on the 

 ground therefore, seldom exceeds the third part of the 

 computed measure, and often will not amount to one-fifth ; 

 and its effects on the air in the shade are probably nearly 

 nugatory, from its increased capacity for caloric, and its bad 

 conducting power. A similar modifying effect is produced 

 by the ground being covered with grass or plants, the 

 multiplied surface of which, exposed to the contact of the air, 

 dissipates the greater part of the heat before it accumulates. 



* This leads to a very important caution on the part of Invalids and 

 others resorting to all mountain regions in tropical latitudes, (at least 

 during the dry season), viz. to expose themselves as little as possible to 

 the direct rays of the sun, without some adequate protection. See 

 Article " Hints to Invalids." 



t Enoyolop. Britannioa, Article, Climate. 

 K 



