PRESIDENTIAL ADDRESS. LIX 



that fully 99 per cent, of the earth's surface temperature is due 

 to heat received by it from the sun, so that heat received from 

 other sources, as from the internal heat of the earth, from radia- 

 tive emanations, from planets or stars, may all be considered as 

 practically negligible. 



Next there is the factor of the different presentment of dif- 

 ferent parts of the earth to the sun's rays at different seasons of 

 the year, due to the obliquity of the axis of rotation of the earth 

 to the path in which the earth revolves around the sun. Next 

 there is the factor of the rotation of the earth from west to east. 

 This factor, as Ferrel has shown, leads to the tendency for any 

 body in the northern hemisphere, whether moving in a north- 

 south direction or in an east-west direction, or in any intermediate 

 direction to be deflected towards the right. In the southern 

 hemisphere the deflection is, ceteris paribtis, towards the left. 



Next there is the factor that the earth's surface is parti 

 formed of land and partly of water, and next that the specific 

 heat of water is about four times that of average land. In other 

 words, land warms up, or parts with its heat four times as 

 quickly as water does. 



Next there is the factor that water, when vaporized, is lighter 

 than air, and thus when present in appreciable quantities in air 

 tends to float the air masses upwards, the molecules of water 

 vapour acting like microscopic balloons, and so when conditions 

 are humid, all other things being equal, atmospheric pressure is 

 less than when the air is dry. 



Next there is the factor that pure normal dry air is practically 

 diathermanous, that is, it allows the sun's heat rays to be trans- 

 mitted through it to the earth without itself undergoing any 

 appreciable rise of temperature. 



Next is the factor that the presence of any of the following 

 substances in the atmosphere lessens its diathermanous properties, 

 viz., water vapour, carbon dioxide, ozone, dust motes. Dust 

 motes may come from ordinary surface terrestrial dust, lava dust 

 from volcanic eruptions, or cosmic dust derived from the volatili- 

 zation and subsequent condensation of meteors and meteorites. 



Next is the factor that if air expands as it rises its heat be- 

 comes distributed over an increasingly larger volume. This so- 

 called adiabatic expansion of air is followed by a fall of tem- 

 perature at the rate of 1.6 degrees F. for every 300 feet of ascent. 



