Hill. — Problems of Arctic Exploration. 393 



the barometric pressure at 30 in. or more. As in the case 

 of water, the atmospheric temperature has a wide range or 

 contrast at the equator, and constantly diminishes its range 

 as the poles are approached. We have no means of knowing 

 what the temperature in the atmosphere would be at an eleva- 

 tion of 20,000 ft. above the polar area, but certainly it would 

 be so low that life such as we are acquainted with could not 

 exist there. 



Each of the zones has a certain quantity of heat-rays 

 available, and no more, and when those rays have done their 

 work on the surface, whether on the land or on the water, 

 they tend to pass back again through the atmosphere, and are 

 subsequently diffused in space. Bat the work which is per- 

 formed by the returning obscure rays in either zone appears 

 to be directly proportional to the radiant heat received from 

 the sun by the zone. And the varying height of the snow-line 

 from the equator to the polar circle seems to suggest the 

 truth of this statement. When the air within the torrid zone 

 ascends, laden as it is with moisture and heat, it has a definite 

 work to perform within the atmosphere of that zone. The 

 difference of elevation in the snow-line of each zone represents 

 the total surplus heat which the atmosphere of the zone has 

 been able to perform owing to the excess or deficiency of heat- 

 rays in the zone compared with the adjacent zone, and I am 

 doubtful whether the heat taken away from a zone by ascend- 

 ing currents is • greater than the heat received in the same 

 zone by the incoming currents. Suppose, for example, that 

 the temperature of the torrid zone were increased by 20°, 

 what would be the atmospheric effect on the other zones? 

 Aerial movements would be more rapid ; evaporation would 

 increase ; heavier rainfall within the tropics would ensue ; 

 the elevation of the snow-line would be raised ; and the 

 seasonal contrasts between the several zones would be more 

 marked than they are now. Of necessity the winds would 

 increase, but the balance would be maintained between lower 

 incoming and upper outgoing currents, no matter what modi- 

 fication in the temperature might take place. 



It has already been explained how wide the difference is 

 between the heat-rays received in the different zones, and 

 when it is remembered that the capacity of the atmosphere 

 for moisture, other things being equal, depends directly upon 

 temperature, it will be seen at once that the vapour from 

 the torrid zone can hardly reach beyond the limits of its 

 own zone, as evaporation in each zone will continue every- 

 where until saturation-point is reached, or, which amounts 

 to the same thing, the natural tendency in each zone is to 

 satisfy the atmospheric and aqueous conditions under the 

 constantly modifying factor of heat. Thus air of high 



