This investigation was undertaken with the view of ascertaining how the mean hourly values of the barometer differed on days in which radiation, solar and terrestrial was great from days in which it was interfered with by clouds. A preliminary examination of the barometric readings at the Ben Nevis and Fort-William Observatories showed that there was a well marked difference between the diurnal fluctuations in these two kinds of weather; and the investigation was thereafter extended to several other places in the northern hemisphere. The materials for this work are only available from comparatively few places, it being necessary to have before us the hourly readings of the barometer for every day, and also an hourly record either of sunshine or of cloud. These two separate data have not been published for any place in the British Islands except Ben Nevis and Fort-William.