236 PROGRESS OP' METEOROLOGY IN 1889. 



if sufficiently uniform and extended, least favorable to its manifestation. 

 A constant moisture-laden westerly breeze would give a climate nearly 

 as clear as that of the southwest corner of France. 



Two principal factors go to the production of ordinary haze 5 the first, 

 a rather large amount of vapor between the earth and a great altitude, 

 say G0,000 feet ; and the second, a mixture of two heterogeneous masses 

 of air. Evidence of the correctness of this proposition is to be found 

 in the geographical distribution of haze and the state of the winds 

 when it occurs. 



In the majority of cases of east wind, and especially when this wind 

 is of brief duration, local or gentle, a westerly wind flows above it at 

 no great distance from the surface of the earth. Considering the per- 

 petual rapid interchanges (hardly to be called diffusion) going on in the 

 atmosphere, the lower wind must be largely mixed with air of a differ- 

 ent condition derived from the westerly current. If a cold, dry east 

 v,ind be permeated by patches and filaments, however minute, of moister 

 and warmer air, they must be cooled by contact with the polar wind, 

 and a slight deposition of vapor may take place. Or the countless in- 

 visible dust particles may, by increased radiation towards space through 

 a drier air, either cause a deposition of moisture upon themselves or 

 collect still smaller particles together, as dust is known to collect on 

 cold surfaces in a warm air. If deposition of moisture take place, the 

 dryness of the air prevents the water particles from growing to any- 

 thing like the size of the particles of a fog; a relatively small diffused 

 quantity of vaporous air in minute parcels could not produce by con- 

 densation any but extremely small and transitory water particles, in 

 the aggregate visible through long distances, but probably individually 

 beyond the power of the miscroscope to discern. They may be com- 

 pared to the blue mist escaping from the safty- valve of a boiler under 

 high pressure, the invisible steam turns for a moment blue and then to 

 the ordinary white of visible steam. The haze may possibly be equally 

 momentary in duration, disolving long before reaching the white stage, 

 but fresh filaments are perpetually keeping up the process and giving 

 the appearance of a persistence like that of smoke or dust. 



The evidence concerning the appearance of haze by irregular trans- 

 mission of light due to unequally heated currents of transparent air 

 seems to be quite insufficient, and however great the heat near the sur- 

 face of the ground, say in the desert, with consequent distortion of 

 images, it does not, as a rule, bring about the haze so common in tem- 

 perate climates. {Nature, xli, p. 60.) 



In a conimujiication in Nature, Mr. J. H. Poynting suggests that com- 

 mon summer haze may be due to local convection currents, which by 

 reason of their difference of temperature and density render the air op- 

 tically heterogeneous. The light received from any object is more or 

 less irregularly refracted, and on account of the motion of the currents 



