In the first part (A) of this paper it is shown that even the latest solar constant observations of the Smithsonian Institution contains a 12-month period, and that its course is exactly the reverse of what it was before the alteration was made in the formula used for the determination of the transparency of the atmosphere. In the second part (B), the changes in the solar constant from 1919–1932, according to Abbot's measurements, are recorded against the sun-spot changes. It seems that the changes in the solar constant are neither parallel to nor opposed to those of the sun spots. But the highest values of the solar constant appear chiefly to occur between the maxima and minima of sun spots, whilst the lowest values occur near the extremes of sun-spot activity. An attempt is made to explain this. In the third part (C) of the work it is pointed out that a similar relationship exists between certain weather phenomena and sun spots.