162 ON THE STORM EXPERIENCED THROUGHOUT THE UNITED STATES 



cending current of December 20th could not, however, exert any direct influ- 

 ence upon the barometer, except near the centre of the storm. To account 

 for the entire oscillation, I think, we must admit another principle quite dis- 

 tinct in its operation. 



10. Let a wind blow ever so violently over the earth's surface, and the 

 diminution of gravity arising from the centrifugal force must be inconsidera- 

 ble. But, imagine the different parts of the current to travel with unequal 

 velocity, and there will arise a mechanical condensation, or rarefaction. When 

 air is at rest, or in motion, with a uniform velocity, its particles are main- 

 tained at a constant distance from each other. But let the velocity of one 

 section be increased beyond that of the succeeding, and the same particles of 

 air are forced to fill a greater space. Such is the principle of the undula- 

 tions which produce the sensation of sound. It appears to me that a similar 

 effect must have been produced in the storm of December 20th. The south- 

 east wind which accompanied the rain moved with an accelerated velocity. 

 The particles, therefore, of air at one extremity of the current must have 

 left those of the other extremity at an increased distance. Hence a mecha- 

 nical rarefaction, and, of course, diminished pressure. The reverse effect 

 must have taken place after the storm had passed. A north-west wind sets 

 in with great violence. A vast body of air is precipitated towards the south- 

 east. The partial vacuum which at first existed is very soon supplied. Yet, 

 though the first impelling cause has ceased to act, the momentum of the 

 excited current still urges it onward. The front of the wave is impelled by 

 the momentum of the mass in the rear, and a mechanical condensation re- 

 sults, bringing, of course, increased barometric pressure. The cause, how- 

 ever, which produces this extraordinary rise, being temporary in its nature, 

 soon ceases, and the barometer falls. The causes which I have here assigned 

 for the oscillation of the barometer appear to me to be such as are known to 

 be true, and that they are sufficient to account for the phenomena. 



I have thus analyzed the phenomena of this somewhat remarkable storm,_ 

 and given such explanations of them as have appeared to me most satisfactory. 

 It is a most interesting and important inquiry, how far the conclusions at 

 which I have here arrived may be safely generalized. I have already made 

 some progress in this investigation; but as the present paper has now 



