LABORS OF THE JOINT COMMITTEE. 141 



the progress of a great storm, it should sometimes snow or 

 rain violently, and at other times stop, with increase or dimi- 

 nution of wind, it might be safely inferred that some such 

 action as that just described is going on. In that case, too, 

 a person below the clouds may sometimes distinguish these 

 cones, which raise their tops above the general level of the 

 cloud above, for their bases will be much blacker than the 

 surrounding clouds. After all, we must wait for future and 

 more abundant facts to explain these irregularities. 



As to the direction in which the storm moved, and its velo- 

 city, we have but little to say, because it is entirely beyond 

 the power of the theory to predict in what direction storms 

 in general will move. It is highly probable, indeed, that very 

 narrow storms of great violence, such as tornadoes, in which 

 the drops of rain are not permitted to fall back through the 

 ascending current, but are thrown outwards, at a great height, 

 frozen into hail, will all be found to move in the direction of 

 the upper current that is, westwardly, or towards the west 

 in the torrid zone, northwardly from the tropic of Cancer to 

 latitude thirty, and north easterly or eastwardly in the lati- 

 tude of Philadelphia. 



For the tornado cloud, forming only when the dew point 

 is very high, that is when the steam power in the air is very 

 great, (for all storms are produced by steam power,) it will 

 rise very high, and of course a large portion of its upper 

 part, being in the upper current of air, it will be pressed by 

 that current in its own direction. Therefore the tornado, as 

 long as it lasts, must move in that direction. But in case 

 the rain falls down through the base of the cloud, as in ordi- 

 nary showers, the descent of the rain produces a disturbing 

 force below, and the accumulation of drops of rain in the 

 cloud prevents the cloud from rising so high into the upper 

 current as in the tornado cloud, and besides the air, on the 

 northern border of the storm being colder and of a lower dew 

 point, will, by its greater weight, have a tendency to press 



