02 C.C. NATURAL HISTORY SOCIETY. 



The atmosphere is twofold, dry air and aqueous vapour. The 

 former remains constant, the latter is always changing, owing to 

 evaporation and condensation always going on. Water, snow, and ice 

 and plants are all of them constantly adding to the aqueous vapour 

 in the air, and since evaporation increases as temperature increases, 

 the capacity of the air is also increased. But this capacity is not un- 

 limited. AVhen, therefore, the air is fully saturated evaporation ceases. 

 Therefore evaporation is greatest when the air is driest. Currents of 

 air remove the moister and substitute drier air, consequently in 

 windy weather evaporation is much more rapid. As air expands 

 under a diminished pressure its temperature consequently falls, and 

 it becomes moister, and as it contracts under an increased pressure 

 its temperature rises and it becomes drier. Hence ascending 

 currents of air become moister as they ascend, and descending 

 currents become drier as they descend. Hence as winds ascend the 

 slopes of hills they become moister, and when they cross the top and 

 flow down the other side, they appear to become drier, although the 

 amount of aqueous vapour in them has remained practically unchanged. 



The elastic force of aqueous vapour is its pressure at the surface 

 of the earth expressed in inches of mercury of the barometer, and is 

 calculated from the temperature of the dew point. Thus the elastic 

 force of vapour indicates the quantity of aqueous vapour in the air at 

 the place of observation, and may be designated the absolute 

 humidity of the air. 



I may here mention that the sudden and violent appearance and 

 action of thunderstorms is believed to be due to the meeting between 

 warm moist currents of air ascending and cold dry descending 

 currents, and where the atmosphere is continuously dry and rainless, 

 or where the pressure is continuously high, thunder is unknown. 



Hail is connected with whirlwinds. The vapour carried aloft is, 

 below a certain height, condensed into rain, above that height, into 

 snow. If, therefore, raindrops formed below are carried up into the 

 wind region they are frozen and fall as hail. They may be drawn in 

 again as they fall and be carried up again, gaining size in the process. 



To ascertain the causes of the various and constant changes 

 in the temperature we must look to another branch of meteorology, 

 viz., the measurement of the pressure of the atmosphere by means of 

 the barometer. Indeed, to changes of the atmospheric pressure 

 are due all the variations in climate to which we are subject; heat 

 and cold, rain and wind, all arise from these changes, and, could we 

 but find out the laws that govern them, we might then hope to see 

 weather forecasting perfected. It is extremely difficult to convey to 



