374 NOTES OF A NATURALIST. 



Heated air rises in the equatorial zone, and its place is filled 

 by currents from the temperate and subtropical zones. The 

 heated air from the equator flows at first as an upper current 

 towards the poles, but as it gradually loses its high temperature, 

 it becomes mixed with the currents setting from the poles 

 towards the equator, causing the atmospheric disturbances and 

 variable winds characteristic of the cooler temperate zones. 

 As a rule, bodies of air of different temperatures do not very 

 quickly mix, but tend to arrange themselves in layers or strata 

 in which masses of unequal temperature are superposed. It is 

 obvious that in such a condition, where a layer of colder air lies 

 between two having a higher temperature, the whole cannot be 

 in a state of equilibrium. But in nature we constantly find that 

 equilibrium is never attained. There is a continual tendency 

 towards equilibrium, along with fresh disturbances which alter 

 the conditions. 



As Professor Stokes remarks in a letter on this subject with 

 which he favoured me, " to know the temperature of the succes- 

 sive strata as we ascend in a balloon, we should know the 

 biographies of the different strata." Those which are now 

 superposed may have been hundreds of miles apart twenty-four 

 hours before. It follows that without a knowledge of the course 

 and velocity of the higher currents existing in the atmosphere, 

 we cannot expect to learn the vertical distribution of temperature. 



Apart from the effects of the great movements of the air, there 

 is another effect of air-currents to be considered, which tends 

 especially to modify the temperature found at or near the 

 earth's surface. The heating of the surface by day, and the 

 cooling by night, determine the existence of local currents of 

 ascending or descending air. In rising, the air encounters 

 diminished pressure, and therefore expands, and in so doing 

 overcomes resistance. The molecular work involved in dilatation 

 is performed at the expense of the other form of molecular work 

 which we call heat. In other words, the air in ascending loses 

 heat. It is found that the amount of decrement of temperature 

 due to the ascent of a body of air is nearly exactly i° C. for loo 

 metres. As a general rule, ascending currents arise from the 

 surfaces exposed to the sun during the day, and must largely 

 contribute to produce the rapid decrement of heat which is 

 found in the lower strata near the surface, as compared with 



