138 PHILOSOPHY OF STORMS. 



of the 17th March, as detailed in the " Report " of the Me- 

 teorological Committee, and summed up in the preceding 

 statement of the facts. 



The upward motion of the air in the region of a storm, 

 may take its commencement either from a higher tempera- 

 ture, or a higher dew point. 



As the air rises in the inner portions of the storm, it is 

 reduced in temperature by diminished pressure, a little 

 more than 1 for every hundred yards of its ascent, as I 

 have ascertained by experiment ; and when it has ascended 

 as many hundred yards as the temperature of the air is 

 above the dew point, the vapor will begin to condense into 

 cloud, and give out its caloric of elasticity this caloric of 

 elasticity is received by the air in contact with the condens- 

 ing vapor, and prevents the air in its further ascent from 

 cooling as fast it would, if there was no vapor in the air to 

 condense ; and, I find, both by calculation and experiment, 

 that in ordinary states of the dew point, it cools only 

 half as much in its ascent above the base or lower part of 

 the cloud, as it would do if no latent caloric were given 

 out; and that in all states of the dew point, the air in 

 the cloud at the moment of its formation, is expanded 

 about six thousand six hundred cubic feet for every cu- 

 bic foot of water generated by the condensed vapor, after 

 making allowances for the condensation of the vapor itself. 



The great expansion of the air in the cloud, will cause a 

 rapid ascent and out-spreading above, which will cause the 

 barometer to fall under the cloud, and if there was no cur- 

 rent above, it would spread out on all sides equally in an 

 annulus, and cause the barometer to rise all round the storm, 

 as much on one side as another. But as there is known to 

 be an upper current always, or almost always, moving in 

 this latitude towards the north east or N. N. E., this cur- 

 rent will cause the out-spreading of the air to be chiefly in 

 that direction, and consequently the barometer will rise 



