EXAMINATION OF REID'S STORMS. 209 



as may be ascertained by looking at a table of dew points. 

 (129). In fact the doctrine of mingling air in the atmosphere to 

 produce cloud, as taught by Dr. Button, will not stand the 

 test of examination, even if a means could be discovered of 

 producing the mixture. For it must be done either by cold 

 air corning down, or warm air going up; now if cold air 

 comes down, it becomes intensely dry, and if warm air goes 

 up, it will condense its vapor by the cold of diminished 

 pressure, and more so the less it mingles with the upper air. 

 Nor is the doctrine that cloud is heavier than surrounding 

 air at the same height tenable, as was shown before. 



The question then resolves itself into a matter of fact ; 

 and a question of great moment it is acknowledged to be ; 

 is there a centripetal or a centrifugal motion of the air in 

 these storms? If the former is true, all the phenomena are 

 explained ; if the latter, nothing is explained ; not even the 

 whirling motion itself. Let the careful reader decide. 



Storm of September 3J, 1821. 



155. The following investigation of the storm of 1821, was 

 written early in the year 1838, and presented to the joint 

 committee of the American Philosophical Society and the 

 Franklin Institute by me, in my official capacity as meteor- 

 ologist of that committee ; but it was not accepted. I now 

 publish it on my own responsibility ; and as this storm has 

 been copied by Col. Reid into his book, it may now be con- 

 sidered as a part of the examination of his law of storms, 

 though it was written before his book, was published. This 

 much it was necessary to say in explanation of the pecu- 

 liar phraseology of what follows, since it is published verba- 

 tim as it was read to the committee. 



PHILADELPHIA, January 10th, 1838. 



GENTLEMEN: As the meteorological instruments (118) or- 

 dered by the Committee have not all been made, and distribu- 



27 



