part and tliat the differences as sIio\vii in these tables ninst be 

 ascribed rather to the nature of I he clouds than to this reason. 



When treated in this way sevei-al peculiarities are observed which 

 remain concealed when daily means are used e.g. the fact that near 

 Helder the cloudiness is smaller than at de Bilt, whereas de Bilt 

 shows 5 7o niore of sunshine. 



Furthermore these tables show that in wintertime also, when 

 the suns elevation is very small (in January for 52° L. N, and 2 

 p. ra. the sun's height is only 12.25°), the sums mostly exceed 100 7o- 



If we do not wish to ascribe the different relations at de Bilt 

 and Helder, characterized by a difference of 10 7o ^^ the sums for 

 the whole year, on the one hand to a difference in the estimate of 

 cloudiness or, on the other hand, to instrumental unequalities, an 

 explanation of these phenomena is required. 



That the sum of cloudiness and duration of sunshine is more than 

 100 7o ^iid that this fact is not shown in monthly means because 

 two opposite causes neutralize each other, has also been remarked 

 by others e.g. by Besson and Coeurdevache '). 



We can now take a further step and, from the frequent occurrence 

 of extreme values, conclude that also the numbers of tables H and 

 HI cannot be considered as a proper measure foi' the relation 

 between cloudiness and duration of sunshine and we may expect 

 that the sums must be considerably larger for average values of 

 cloudiness. 



This conclusion is fully confirmed by the data of tables IV and 

 V, giving the results of a classification of percentages of sunshine's 

 duration according to those of cloudiness. Table V is printed as 

 an example of the way in which different combinations may occur. 



Although the numbers of table IV do not everywhere show a 

 regular course, they clearly demonstrate that the sums of cloudiness 

 and duration of sunshine may amount in summer to more than 

 130, in winter to more than 110 7o- 



In this communication the disagreements between different stations 

 are not taken into consideration and the question is restricted to 

 the problem : how to explain the absolute values, the differences 

 between summer and winter and the position of the maxima as shown 

 in table IV for the station de Bilt only. 



2. In order to find a possible relation between the results of 

 both methods of observing the covering of the sky, it is in the first 

 place necessary to consider, what is understood by the term "degree 



1) Ann. Soc. Meteor, de France, 56, 1908 (73). 



