128 Report S.A.A. Advancement of Science. 



laborious process could be used in getting the values under cloudy 

 skies, albeit there are other and shorter ways. The most suitable 

 seems to be : 



At any hour in a given month 



Let p be the pres.sure (.say) under cloudy skies ; 



P the j)ressure under clear skies; 



IT the pressure under all skies. 



Let there be a cloudv days in the month and .4 clear; 



so that k=a^ A. 



Then ap + AP=(;i + A) - 



.-. ap + (k — ;i) P = k;r 

 .-. P = P— (ka) (P — tt). 



This simple formula is so powerful withal that given a five 

 years' full register, and the values under clear skies, then the monthly 

 sets of hourly values of any element — temperature, pressure, 

 humidity, etc. — may be obtained completely in about two hours. 

 Here, again, the yearly means are the simple average of the monthly 

 sets. 



There are a great man\ mure clear days in winter than in 

 summer, sa that if we were to consider the year of clear or cloudy 

 days to be the average of the days, then the annual mean diurnal 

 curve under clear skies would display winter characteristics, while 

 the curve under cloudy skies would display those of summer. Bv 

 considering the yearly means to be the average of the monthly sets 

 we eliminate the effect of the unequal distribution of days. And. 

 therefore, we get a representation of a very clear or a very cloudy 

 year. But of course the mean year will not l)e the arithmetic mean 

 of the two components. 



Table 14 gives a comparative view of the harmonic constituents 

 as far as the fourth term. 



