26 Mr. R. M. Deeley on the 



than that resulting from the surface temperature gradients, 

 is shown by the low pressure area of which the Antarctic 

 Continent is the centre, and the two low pressure areas in the 

 Northern Hemisphere. 



In the case of the curves of temperature and pressure we 

 have illustrated, only the north and south components of the 

 winds are indicated by the arrow-heads. The winds, how r - 

 ever, seldom blow north and south, and they always show the 

 effects of the rotation of the earth; but the direction of the 

 winds is not, as we have seen, in very many important 

 instances such as the surface temperature gradients would 

 suggest. 



When Halley and Hadley propounded their views nothing 

 was known of the changes of temperature which take place 

 with elevation in the upper atmosphere, and it has generally 

 been assumed that the temperature gradients which produce 

 the general circulation of the winds of the world are those 

 which are shown by the temperature gradients at the earth's 

 surface. 



It is to Teisserenc de Bort that we owe the discovery that 

 the temperature of the upper atmosphere presents some 

 remarkable features. He found, by means of registering 

 balloons, that at a height of about eleven kilometres the 

 temperature of the air ceases to fall, and sometimes even 

 becomes warmer with increasing height. The lower eleven 

 kilometres, or thereabouts, where, owing to the vertical 

 temperature gradients, vertical currents can take place, has 

 been called the troposphere, while the upper portion, where, 

 owing to the very slight increase or decrease of temperature 

 with varying height, vertical currents of any strength cannot 

 exist, has been called the stratosphere. 



This peculiar distribution of temperature with vertical 

 height is apparently due, as pointed out by Gold *, to the 

 fact that the sun's heat is pretty regularly intercepted by the 

 whole thickness of the atmosphere either on its way to the 

 earth's surface, or as it is again radiated from the earth and 

 air into space. Indeed, the upper portions of the atmosphere 

 are pierced by all the heat the earth receives from the sun 

 and by the same amount of heat as it is returned to space. 

 On the other hand, the lower portions of the atmosphere 

 receive a good deal of heat owing to convection currents rising 

 from the earth's surface. This ground, or water-heated air, 

 cools as it expands on rising, and a condition of convective 

 equilibrium is set up in the lower atmosphere. The actual 



* Proc. Roy. Soc. A. vol. lxxxii. 



