aiETEOEOLOGY AND ALLIED SUBJECTS. 275 



S sliow tliat even at zero degrees altitude the optical iutensitj of 

 direct sunlight is much greater than the optical intensity of diffuse 

 light. It does not follow from this that the chemical iut<'nsities have 

 no relation to the optical, for the chemically active rays are almost en- 

 tirely absorbed in the atmosphere when the sun is low ; aad we must 

 conclude that the optical and chemical intensities of the snnlight vary 

 differently with the solar altitude. Again, this table shows us that up 

 to a considerable altitude, say 31°, the chemical intensity of diffuse day- 

 light is greater than that of direct sunlight. This result is confirmed 

 by the observations of the photographic actinometer. We here per- 

 ceive another beneficial influence of our atmosphere similar to that 

 experienced in the case of heat and light. 



In northern regions where the altitude of the sun never exceeds about 

 30°, the chemical intensity of the direct sunlight will have too feeble an 

 effect; and the effect of diffused light compensates for this. Thus at St. 

 Petersburg at the time of equinoxes the chemical intensity of diffuse 

 daylight is twice as great as that of direct sunlight, and at Melville 

 Island it is ten times as great. 



The preceding data give us the means of computing approxiDiately 

 the total amount of absorption that must take place in the atmosphere 

 of the earth, and it results that an intensity of 35.3 units at the outer 

 surface of the atmosphere becomes 20.0 at the earth's surface. This im- 

 mense absorption is surprising and is unequally di\ided between the 

 various portions of the spectrum. The chemical end loses about one- 

 third; and the warm rays of the spectrum lose about thre«-fourths of 

 their original intensity. 



The influence of the cloudiness is in every respect the most disturb- 

 ing. This has been studied by Stelling, who concludes that the influence 

 of a partial cloudiness is now to raise and then to lower the absorjition. 

 Second, the influence of partial cloudiness, if the sun is behind clouds, is 

 almost always without exception a depression, and on the average about 

 30 per cent. Third, the influence of a completely covered uniform gray 

 sky is still more depressing; it lowers the normal intensity on the 

 average by more than one-half. (Z. 0. G. M., XIV, pp. 401 to 426.) 



In an essay on the ripening of ftniits, Levy investigates the manner 

 in which the insolation affects the development of the plants. It is evi- 

 dent that wo must not only from agricultural reasons, but also in the 

 general interest of the science in the future, lay more stress on the inso- 

 lation observations ; but whether the Arago-Davy instrument is to be 

 adopted as sufiicieutly safe cannot be decided here. It must be decided 

 first of all what rays of the solar spectrum produce these chemical effects, 

 and in case these are not the brightest, then this actinometer does not 

 measure the heat they send ; but it does serve excellently as a photo- 

 chemical method. (Z. 0. G. ilf.. Vol. XV, 1880, p. 30.) 



Whipple has discussed the measurements of relative durations of sun- 



