4 PHILOSOPHY OF STORMS. 



the cloud is formed is prevented from contracting as much 

 as it would otherwise do, by the latent caloric given out at 

 the moment of the condensation of the vapor, and that too 

 exactly in proportion to the quantity of vapor condensed. 



11. The velocity of the cloud upwards will therefore de- 

 pend on its perpendicular depth and on the height of the 

 dew point; for on this last will depend the quantity of 

 vapor condensed during the upward motion of the air. The 

 theory will best be illustrated by calculating a particular 

 case. 



12. Suppose the dew point at 71, when by article I the 

 quantity of vapor in the air at the surface of the earth is 

 ^ of the whole weight. Suppose also the temperature to 

 be 75, or 4 above the dew point; suppose a column to be- 

 gin to rise either from superior heat or superior moisture : 

 and suppose an extreme case, unfavorable to the theory, that 

 the column in ascending cools by expansion one and a half 

 degrees for every hundred yards of ascent, while the atmos- 

 phere around the column is only one degree colder for a 

 hundred yards ; the effect will be, that the column will 

 ascend only a little more than three hundred yards when 

 some of its vapor will begin to condense. Now to ascertain 

 what its temperature shall be at any particular height, sixty 

 hundred yards for instance, we have only to find a point 

 below 75, at which sufficient vapor will be condensed to 

 heat up the air as many degrees as this point wants of being 

 one degree and a half below 75, for every hundred yards of 

 ascent, or in the present case 90. For as in this case the 

 air is supposed to fall in temperature 90, in ascending sixty 

 hundred yards, there is nothing to prevent its falling this 

 quantity but the latent caloric evolved in the condensation of 

 the vapor. Now by examining a table of the dew point (129), 

 according to Dalton, it will be found that if the temperature 

 falls 48, it will, after making allowance for the increased 

 space it occupies, condense f of its vapor, sufficient to heat 



