PRP:SIDENTS ADDRESS. 31 



to the rate of travel of the body, and thus furnish a means of 

 telling the direction and speed of motion. It is further evident 

 that waves beyond the visible spectrum at one end or the other 

 may be brought into the visible range by acceleration or retarda- 

 tion. So also, rays belonging to the visible red end may V)e 

 changed into invisible heat, or others at the blue end may be 

 pushed forward and accelerated into invisible chemical rays. 



The temperature of a heat-emitting body has a direct influence 

 on the nature of the heat evolved. The hotter the body the more 

 nearly the heat waves approach the properties of red light, 

 while the cooler the body the more they tend to become 

 obscure. All heat given off from a body of the nature of the 

 earth is of the obscure type. Radiant heat is not reflected as 

 .such but is degraded and sent away in the lower form. 



We may now picture the earth's atmosphere, containing carbon 

 dioxide and water vapour, with the sun's radiant energy pouring 

 into it. The long waves of obscure heat will be absorbed in the 

 upper layers of the atmosphere, leaving the shorter waves of 

 radiant heat to pass on and reach the surface of the earth, where 

 they are at once absorbed, partly by the solid and partly by the 

 aqueous surface. The warmed solid surface proceeds to part with 

 its heat by radiation and contact with the air, the heat emitted 

 being now of the obscure type, and so unable to pass the carbon 

 dioxide and water barrier. The result is that the lower layers 

 of the atmosphere become warmed by the transformed heat 

 which before passed freely through. The fate of the heat taken 

 up by water is mainly to cause evaporation, whereby it is 

 carried in the latent state in the water vapour and liberated where 

 the vapour condenses to form clouds, the ultimate result being, 

 that like that absorbed by the land, it goes to warm the 

 atmosphere. Finally the heat acquired by the atmosphere is scat- 

 tered in all directions, some back to the earth, some laterally to the 

 surrounding portions of the atmosphere, and some into space. 

 A condition of equilibrium is then established, the earth losing 

 heat at the same rate as it is receiv^ed, but with the vastly 

 important provision that its own surface remains at a tempera- 

 ture high enough to give what I might term heat pressure sufii- 



