14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. OO 



days. In this way be has divided his measurements of nocturnal 

 radiation into two groups, one group being 'taken on the nights of 

 the days in which the transparency of the atmosphere had been 

 above the average, and the other group taken on the nights of the 

 days in which the transparency of the atmosphere had been below 

 the average. The number of days in each group is 17. He finds 

 that the average nocturnal radiation was about 0.15 calorie per 

 square centimeter per minute, and that the nights corresponding to 

 days of more than usual transparency of the atmosphere gave a 

 nocturnal radiation 0.001 calorie above the normal, while those 

 nights corresponding to days of less than usual transparency gave 

 a nocturnal radiation 0.001 calorie below the normal. This differ- 

 ence, 0.002 calorie, between the hazy days and the clearer ones is 

 so small that Mr. Angstrom is doubtful if it be a real effect, or only 

 an accidental error of measurement. 



However, it would not be expected that the effect due to a differ- 

 ence of haziness between the two groups of days would be so great 

 as the effect due to the haziness produced by the volcano ; and it can 

 also be shown that the effect which might be expected from the 

 volcano itself would not be very great, measured by thousandths of a 

 calorie. 



It is shown by us * that the direct radiation of the earth to space 

 is not, perhaps, greater than 10 per cent of the total radiation of a 

 body at the temperature of the earth. Mr. Angstrom's measure- 

 ments incline him to think that the transmission of direct radiation 

 to space is somewhat greater than this, perhaps 15 or 18 per cent. 

 Suppose that we should assume it to be the latter. A perfect radiator 

 at the temperature of the earth emits about 0.5 calorie per sq. cm. 

 per minute. Taking 18 per cent of this we have 0.09 calorie. Then 

 we assume that of the 0.15 calorie representing the nocturnal radia- 

 tion, 0.09 calorie would be transmitted from the earth's surface to 

 space and the remaining 0.06 calorie would be the counter-radiation 

 of the cooler atmosphere towards the earth. Let us further suppose 

 that the volcanic haze (which produced a decrease, as we have found, 

 of 10 per cent in the incoming energy) produced a decrease of 5 per 

 cent in the transmission of the higher atmosphere to the radiation 

 sent out by the earth. This effect of course will influence Mr. 

 Angstrom's results only on the 0.09 calorie supposed to be trans- 

 mitted from the soil through the atmosphere to space. Five per 

 cent of o.cx) calorie is 0.0045 calorie ; so that if it should be found 

 that the nocturnal radiation experienced a decrease of 0.0045 calorie 



1 Annals Astrophysical Observatory, Vol. 2, pp. 167-172. 



