234 THE WEATHER 



mercury barometer. It consists of a metal box from which the air 

 has been removed. One side of the box is connected by a system of 

 levers with a pointer that shows the atmospheric pressure. When the 

 pressure increases, the side of the box is pushed inward, and the pointer 

 moves forward. When the pressure becomes lower, the side of the box 

 springs back into place, and the pointer moves backward. 



264. Heating of the Air. The final source of the heat 

 of the earth's surface is the sun (cf. 76). The sun's 

 rays give up part of their heat to the atmosphere as they 

 pass through it. But a much larger part of the sun's 

 heat reaches the earth's surface. This portion is partly 

 reflected back into the atmosphere and into space, and 

 partly absorbed (cf. 173 and 176) by the earth. The 

 part absorbed warms the land and the water for a time; 

 afterwards it is given back to the air. The total amount 

 of heat absorbed by the atmosphere has been estimated to 

 be about 70 per cent of the amount received from the 

 sun. Of course this absorption is only temporary; for in 

 the end the quantity received and the quantity lost are 

 practically the same. 



The lower part of the atmosphere absorbs heat much 

 better than the upper part, and becomes warmer. The 

 reasons for this are that the lower air is denser, and that 

 it has more impurities : dust, smoke, clouds, carbon diox- 

 ide, and water vapor. All of these enable the atmosphere 

 to hold its heat. It is these good absorbers that make 

 the atmosphere a trap for the sun's energy, and a blanket 

 to prevent too rapid heating and cooling (cf. 183). 



Air is also heated and cooled by its own contraction and expansion, 

 respectively (cf. 71). As it ascends, and therefore expands, it 

 becomes colder. This is the reason why ascending air currents finally 



