In this way he established the fact that the wind, far from being a body of even approximate uniformity, is under most ordinary conditions irregular almost beyond conception. Further, that the greater the speed the greater the fluctuations, so that a high wind has to be regarded as "air moving in a tumultuous mass," the velocity at one moment perhaps forty miles an hour, then diminishing to an almost instantaneous calm, and then resuming. "In fact, in the very nature of the case, wind is not the result of one simple cause, but of an infinite number of impulses and changes, perhaps long passed, which are preserved in it, and which die only slowly away." When we come to take observations of temperature we find the conditions in the atmosphere above us to be at first sight not a little complex, and altogether different in day and night hours. From observations already recorded in this volume--notably those of Gay Lussac, Welsh, and Glaisher--it has been made to appear that, in ascending into the sky in daytime, the temperature usually falls according to a general law; but there are found regions where the fall of temperature becomes arrested, such regions being commonly, though by no means invariably, associated with visible cloud. It is probable, however, that it would be more correct not to interpret the presence of cloud as causing manifestation of cold, but rather to regard the meeting of warm and cold currents as the cause of cloud. The writer has experimented in the upper regions with a special form of air thermometer of great sensibility, designed to respond rapidly to slight variations of temperature. Testing this instrument on one occasion in a room of equable warmth, and without draughts, he was puzzled by seeing the index in a capillary tube suddenly mounting rapidly, due to some cause which was not apparent, till it was noticed that the parlour cat, attracted by the proceedings, had approached near the apparatus. The behaviour of this instrument when slung in the clear some distance over the side of the balloon car, and carefully watched, suggests by its fitful, sudden, and rapid changes that warmer currents are often making their way in such slender wandering rills as have been already pictured as permeating the broader air streams. During night hours conditions are reversed. The warmer air radiated off the earth through the day has then ascended. It will be found at different heights, lying in pools or strata, possibly resembling in form, could they be seen, masses of visible cloud. The writer has gathered from night voyages instructive and suggestive facts with reference to the ascent of air streams, due to differences of temperature, particularly over London and the suburbs, and it is conceivable that in such ascending streams may lie a means of dealing successfully with visitations of smoke and fog.