134 MR. JOSEPH BAXENDELL 



curve No. 4. The first maximum still occurs in April, 

 but a slight change has taken place in the time of the 

 second, which now occurs in the latter part of August 

 instead of the middle of September; but no change has 

 taken place in the times of the two minima. 



Finally, taking the ten lowest values in each month, we 

 have curve No. 5. Here the first maximum has almost 

 disappeared, though still occurring in April, and the second 

 maximum has advanced into July. The principal minimum 

 is still in December, but the secondary minimum occurs 

 in May instead of June, though, like the first maximum, 

 it has almost disappeared. 



It seems, therefore, from a comparison of these five 

 curves, that the general curve in diagram A, which is laid 

 down from the monthly means of all the observations, 

 may be regarded as compounded of two primary curves, — 

 one having two well-marked maxima in April and Sep- 

 tember and two minima in December and June, and the 

 other having only one maximum in July and one minimum 

 in December. From curves 3 and 4 it appears that the 

 influence of the second primary curve upon the general 

 features of the first does not become apparent while the 

 number of selected days is limited to five per month. 

 The first primary curve therefore represents the monthly 

 changes in the calorific intensity of the sun^s direct rays 

 on cloudless or nearly cloudless days, and it leads us to 

 this remarkable conclusion, that the heating power of 

 direct sunlight on clear days in the latitude of the British 

 Islands is greater in the months of April and September 

 than in the month of June, when the sun attains his greatest 

 meridian altitude. 



The second primary curve presents the intensities when 

 the solar rays are more or less intercepted and dispersed 

 by clouds and haze, and it approaches in form the annual 

 curve of temperature ; but the first curve, of which curve 



