492 



SCIENTIFIC RECORD FOR 1883. 



the blue sky is replaced by a white or opaline tint, and the total amount 

 of light and heat thus diffusely reflected to the observer is one-half as 

 much as that directly coming from the sun. The bluest skies are in- 

 dicative of the absence of moisture, the reddest skies are apparently due 

 to vapors or exceedingly fine particles of moisture or of dust, the white 

 sky to the larger aqueous particles that are apparently of the size of the 

 finest cloud or fog particles. 



(6) The number and extent of the sudden changes from warm and 

 moist to cold and dry, or, vice versa, cold and dry to warm and moist, as 

 distinguished from the general variability of these elements (shown for 

 example in the next paragraph, without regard to the direction in which 

 the change occurs. A few words on this subject by the author will be 

 found in the Sanitary Eecord for July, 1879.) 



(7) The frequency and extent of changes in the mean temperature of 

 successive days ; this is held by Hann to be the best way of express- 

 ing the effect upon mankind of variability of temperature, and on page 

 504 he gives the following valuable comparative table : 



Variability of mean daily temperatures, as shown by the frequency per month of 30 days of 

 the occurrence on successive days of mean temperatures that differ by definite amounts. 



