BELL. — SOME METEOROLOGICAL USES OF THE POLARISCOPE. 411 



shows the presence of much aqueous vapor, indicates the progress of 

 nucleation. 



On several occasions I noted this phenomenon in the Breezy Point 

 observations. Starting with strong polarization on the distant hills to 

 the southward and a strong rain band visible in the spectroscope, the 

 next few hours showed a conspicuous weakening of the polarization, 

 followed presently by the formation of visible clouds, and in at least 

 two cases by precipitation. In short, if from change of temperature or 

 other cause cloud is due to form in any particular direction, the nuclea- 

 tion which precedes visible fog formation is bound, other things being 

 equal, to cut down the polarization. The prognostic value of this pro- 

 cess depends largely upon the rate at which it progresses. In two 

 instances which I noted, the decrease toward the south occupied most 

 of an afternoon. Of course a drifting in of coarser dust particles would 

 produce weakening of polarization, but the concurrence of weaken- 

 ing with a heavy rain band intimates very strongly that nucleation is 

 progressing. 



A detailed study of the changes would require the use of a sensitive 

 polarimeter, by which variations from the theoretical polarization could 

 be accurately measured. Observations of this kind, made where there 

 is a wide sweep of horizon, should frequently disclose incipient cloud 

 formation and the causes which produce it. The use of a spectro- 

 polarimeter would be very desirable, as showing by the change in the 

 quality of the scattered light the progress of events. The nature of 

 the minute nuclei, whether dust or water particles, is not definitely 

 known. After a heavy rain storm the lower strata seemed to have 

 been cleared pretty effectively of polarizing nuclei, while the upper sky 

 remained much as before. On one occasion, more than twenty years 

 ago, I was taking rain band observations on Moosilauke and was favored 

 with a day in which the distant peaks, even up to one hundred miles, 

 stood out almost as black as silhouettes, while the sky took on a deep 

 hue almost startling in its unfamiliarity. A polarimeter would cer- 

 tainly have given extremely interesting results had it been at hand. 

 It seems quite possible that one might get a fairly clear idea of the 

 relative number and distribution of nuclei in the upper air by such 

 means. 



It would certainly be interesting also to find out whether the appar- 

 ently very strong absorption of ultra-violet rays by the atmosphere is 

 due to any genuine absorption or merely to a serious loss of light by 

 lateral scattering, which Rayleigh has shown may perhaps be due to 

 the air molecules themselves. In the lower strata my observations 

 pointed rather to dust than to minute water nuclei, since a whitish 



