METEOROLOGY. 107 



it off to some other district of country. But when these disturbing 

 causes are feeble, as when the air is nearly calm, and the daily change 

 of temperature is inconsiderable, the gradually increasing moisture, in- 

 stead of being suddenly condensed into ordinary cloud, begins to con- 

 dense in the form of exceedingly minute particles. These at first only 

 lightly destroy the transparency of the sky, but as they increase in 

 number and size, they render it more dusky until, at length, they very 

 much resemble an ordinary fog. 



If we suppose the temperature and pressure to remain unchanged 

 for only a month or two, the effect would become most striking and ex- 

 traordinary. There would be no horizontal or vertical currents to form 

 ordinary cloud and rain, and thus to withdraw the moisture from the 

 air. The vapor, being constantly increased by evaporation from the 

 surface of the earth and ocean, would first form the thin cloud, scarcely 

 destroying the transparency of the air, then the dense smoke of Indian 

 Summer, and then, when the point of saturation would have been reach- 

 ed, an uninterrupted night of ordinary cloud. That the obscurity du- 

 ring the Indian Summer, or any of the other periods of similar weather, 

 is not greater than it is, especially when it continues for two weeks, is 

 owing to the fact that the equilibrium of forces is not perfect, but only 

 comparative, and that the atmospheric movements, though feeble, are 

 constantly interfering; and that there is much less obscurity at one 

 time than another, is owing to the greater activity of the disturbing 

 forces. The whole year is but a struggle between the efforts of the in- 

 visible vapors to aggregate themselves together in such a manner as to 

 hide the face of the heavens entirely and continuously, and of the at- 

 mospheric movements to disturb and break up such a state of things. 



It is during such times, viz : when the air is slightly obscured by 

 this thin vail of vapor,that the phenomenon of the "diverging beams," 

 or, as it is vulgarly called, " the sun drawing water," may be seen. — 

 When there are small masses of clovd floating above, through the in- 

 tervals of which the sun's rays may pass towards the earth, the parti- 

 cles of vapor intercepting and reflecting a portion of light, enable us to 

 see them in luminous lines, in the same manner that we see illuminated 

 lines of particles of dust floating in a room into which the light of the 

 sun has been admitted by a window. The " diverging beams," indicate 

 the existence of increasing moisture in the lower strata of the atmos- 

 phere, and are not without reason regarded as a sign of approaching 

 rain. 



An interesting example illustrative of our subject occurred during 

 the month of March, 1844. For four days, beginning with the 25th, 



