63 The Philippine Journal of Science ' i9u 



Observations of the radium-emanation content taken during, 

 or immediately after, a period of rainy weather show a compara- 

 tively low value, while those corresponding to the periods of 

 fair weather are almost invariably high. This is especially 

 noticeable during the months when rainy and fair periods alter- 

 nate. It is regretted that until the present time no observations 

 have been taken during an exceptionally heavy typhoon, when 

 extremely low values might be expected.' ' In the months of the 

 dry season, when the precipitation is very light, there seems to 

 be a tendency for the emanation content to decrease. This may 

 be due to a certain extent to the prevailing direction of the wind 

 during that season or possibly to the comparatively high values 

 of the total wind movement during the time of exposure. A 

 high value of the daily wind movement is generally associated 

 with a low value of the emanation content, although this effect 

 is partly masked by the fact that heavy rains and a high wind 

 velocity generally go hand in hand. No correlation is noticeable 

 with a rising or falling barometer, which is contrary to the con- 

 clusion drawn by Simpson " from observations taken in Lapland 

 in 1906 by the active-deposit method. Simpson also found that 

 the radioactivity of the atmosphere increases as the humidity 

 increases and vice versa, but if such a relation exists it is masked 

 in our determinations by other factors. It is very likely, how- 

 ever, that the effect observed by Simpson by the active-deposit 

 method is due to a change in nucleation of the atmosphere rather 

 than to a variation in the radium-emanation content. 



In the above discussion of the variation with meteorological 

 changes we omitted the observations taken entirely during the 

 daytime, for it was found that the strictly day exposures gave 

 much lower values on the average than those extending through- 

 out the night. In Table VIII the day and night observations are 

 compared. 



" Since writing the above discussion we have obtained determinations 

 during typhoon weather which show much lower values than any previously 

 obtained for Manila. 



'* Phil. Trans. (1905), A, 205, 61-87. 



