62 



OK THE OBIGIN AND NATURE OP 



of 70° and of the density and weight which belong to that 

 temperature, will have a tendency to press under and force 

 up the adjoining column that has the higher temperature of 

 75°, and which is therefore proportionately lighter ; and the 

 heavier column will press up the lighter with a force equal to 

 the difference in the weights of the two, which is expressed 

 in the numbers of the table by 5° of temperature. At the 

 height of two thousand yards the difference of temperature in 

 the two adjoining columns is 10°, and consequently the clear 

 air at this height will have a tendency to press up the recently 

 clouded air with a force expressed by the 10°. At three 

 thousand yards high the superior weight of the clear air is 

 15°, and at four thousand eight hundred yards, when the 

 freezing point in the clear air is reached, the difference in the 

 two columns is no less than 24"^. Thus we see, that on con- 

 densation taking place in any particular part of the atmos- 

 phere where the temperature and dew-point at the surface 

 were at 80^, it would make that part so light as to permit it 

 to be forced up by the adjoining heavier air at an increasing 

 velocity, expressed by the numbers in the table which indi- 

 cate the differences of the temperatures at the various heights. 

 The commencement of this process would be slow, like the 

 first movement of a railway carriage by a steam engine, but 

 the velocity of the ascending current would increase with the 

 difference of the temperatures of the two columns, until the 

 aqueous vapour, the material furnishing the moving power, 

 was exhausted. And as the velocity of the ascending cur- 

 rent increased, so would the quantity of air that ascended 

 within it increase ; and the greater that increase the larger 

 would be the quantity of the atmosphere that would press 

 from adjoining parts, to fill the comparative vacuum that had 

 been made by the condensation of the vapour. Here then 

 we see, that under the circumstances described, a very ener- 

 getic expanding power comes into action in the atmosphere, 

 which reduces the weight of the air in the locality, whilst the 



