6 J. Bridges Lee — A peculiar atmospheric phenomenon. [Jan., 



u nder which the phenomenon appears. First, it may be noticed that for 

 a succession of days we have had an absolutely unclouded sky. Secondly, 

 there has not been for some days a breath of wind to stir the atmosphere. 

 Now during all this time steady evaporation has been going on from the 

 surface of the plains and the aqueous vapour in obedience to the ordinary 

 laws of gaseous diffusion has been steadily diffusing upwards through the 

 atmosphere, so that the state of relative saturation of the upper layers of 

 the atmosphere has been steadily approaching the state of saturation of 

 the layers immediately below. Now, in this condition of affairs, when the 

 sun dips towards the western horizon, and the cooling effects of radiation 

 commence to preponderate over the heating effects of the sun's rays, the 

 upper strata of aqueous vapour will be cooled to the condensation point 

 before the lower layers. The reasons why this should be so will be clear 

 to any person who has sufficiently considered the laws of radiation and 

 selective absorption. The layer in immediate contact with the earth will 

 form the only exception to the general rule, which otherwise will apply 

 to the whole thickness of the atmosphere. The earth as a solid body 

 radiates heat of every degree of refrangibility within a very wide range, 

 the upper limit of which appears to stop short of the visible spectrum. 

 Aqueous vapour, as well as the gases and vapours which mixed together 

 form the atmosphere, exercise selective absorption as to certain portions 

 of this radiant energy which chance to synchronise with the several vibra- 

 tory periods of their several molecules, but the remaining vibrations all 

 pass freely through the air which is for them a perfectly diathermanous 

 medium. It follows that the outer and exposed surfaces of the earth get 

 rapidly cooled by radiation, and the layers of air in immediate contact 

 with such surfaces get cooled by conduction. If the surfaces of contact 

 are below the temperature of condensation which corresponds with the 

 then hygrometric state of the atmosphere in immediate proximity to 

 those surfaces, dew is deposited and subsequently fresh vapour diffuses 

 downwards, and is in its turn condensed and so on. These obseiaiations 

 apply only to layers in the immediate neighbourhood of the earth. At 

 greater heights the conditions are quite different. There all heat vibra- 

 tions radiated from water surfaces and all radiated vibrations from solid 

 surfaces which synchronise with the vibratory periods of the molecules 

 of water will be arrested by those molecules and the effect will be to tend 

 to maintain them in the gaseous state. This effect will be progressive and 

 will be greatest at lower levels and least at higher levels. Now besides 

 the radiation from the earth which we have been considering there is 

 radiation from the atmosphere and from every particle in it. Radiation 

 from the upper layers is least checked by selective absorption and the 

 upper layers are also least heated by radiant energy from below, so that 



