56 REPORT — 1841. 



when they overspread the earth at a moderate elevation, seems to accom- 

 modate itself to all the variations of form in the subjacent soil." Mr. Green 

 has also found, that it is usual to ascend to a greater elevation to experience 

 the same reduction of temperature when the earth is overspread with clouds 

 than in a cloudless sky. According to Mr. Monck Mason, a singular rela- 

 tion is found to exist between the formation or precipitation of rain, and the 

 condition of the sky above the clouds which contain it. " Whenever from a 

 sky completely overcast with clouds rain is falling, a similar range of clouds 

 invariably exists- in a certain elevation above, whereby the rays of the sun 

 are intercepted from the layer below ; and on the contrary, whenever, with 

 the same apparent condition of the sky below, rain is altogether or generally 

 absent, a clear expanse of firmament, with a sun unobstructed by clouds, is 

 the prevailing character of the space immediately above : thus leaving it a 

 determinate fact, that when rain is pouring from clouds overspreading the 

 earth, the rays of the sun are not operating upon the clouds in question ; 

 while, on the other hand, rain does not fall from such clouds when the rays 

 of the sun are unobstructedly falling upon the upper surface." According 

 to the same authority, and in conformity with the opinion of Mr. Green, it 

 appears that, in this country, whatever may be the direction of the wind 

 below, in the higher regions, that is, generally within 10,000 feet above the 

 surface of the earth, the direction of the wind is invariably from some point 

 between the north and west. It appears from Mr. Green's observations, that 

 " the variation experienced in the course of the wind during the progress of 

 the ascent was accompanied by a corresponding alteration in the intensity of 

 its rate, the current which at the commencement was gentle becoming strong 

 as it took another direction, and vice versa." These important facts in 

 Meteorology could not have been ascertained by any observations made at 

 the surface of the earth, and afford strong evidence of the advantages which 

 might result to science from well-planned aeronautic expeditions. With 

 regard to the atmosphere of vapour, it is probable that it tends to the main- 

 tenance of an analogous but very different progression of density and tem- 

 perature, from below upwards, to that of the gaseous atmosphere ; but being 

 constrained to diffuse itself through the latter, it is controlled and regulated 

 by the temperature into which it is thus forced. Thus the elasticity with 

 which it will rise from the surface of the earth, in the act of evaporation, 

 will be determined by the temperature of some upper stratum of the air, at 

 which it will become condensed, the force at which point will limit by its 

 reaction that of the evaporating surface. Between these two points, there- 

 fore, the dew-point will probably be found to be steady, or to decline by a 

 very slow progression. After passing through the cloud, it may be expected 

 that the dew-point will fall at once several degrees ; the elasticity of the 

 vapour on the upper side being probably governed and determined by a new 

 point of condensation in still higher regions, just as the dew-point on the 

 surface of the earth is conceived to be determined by the temperature of the 

 first vapour-plane. This would imply, that while precipitation was taking 

 place on one side of a bed of clouds, rapid evaporation might be going on 

 upon the other. It is also conceivable that these processes of condensation 

 and evaporation may be so adjusted as that they may exactly counteract each 

 other ; and the vapour-plane might thus be indicated by no cloud, or possibly 

 by a mere haze ; but the dew-point would fall suddenly. To this circum- 

 stance the observer's attention should be particularly directed. It is probable 

 that, in ascending to a great height, several vapour-planes might be thus 

 crossed, and the confirmation of the hypothesis would be of importance to 

 science in elucidating the constitution of the atmosphere. It is obvious that, 



