2 10 THE POPULAR SCIENCE MONTHLY. 



iiig curreut of air. This involves two other functions: 1. A rushing 

 in of tlie air at the under part of the ascending current ; 2. An 

 outrushing at the upper. Upon the former of these functions, com- 

 bined Avith modifying circumstances, depend the peculiar character 

 and career of the under-currents and of the clouds they bear ; upon 

 the latter, combined with the same circumstances, the proportions and 

 direction of motion of the upper currents and of the heavy masses of 

 clouds they bear. There appears to be nothing in the nature of the 

 cyclone itself which can determine the motion of either the upper or 

 under current more toward any one point of the compass than toward 

 the others. This direction of motion relative to the ascending column 

 depends upon the direction and velocity of motion of the latter, and 

 of the attnosplieric strata in which the influx and efflux take place, 

 modified to some extent by the difiering velocities of revolution of the 

 surface of the earth at different parallels of latitude, by the form of 

 the earth's surface, and by the variation in the constitution of the 

 atmosphere. If the cyclone column and the atmospheric strata which 

 it penetrates move in the same direction, and with the same velocity, 

 the influx and efflux will take place in nearly equal quantity, on all 

 sides of the column. If they move with different velocities, the direc- 

 tions of exaggeration and diminution of the influx and efflux can be 

 calculated in the same way as the direction of a vane on a ship's mast, 

 given the directions and velocities of the motion of the wind and of 

 the ship. 



This gives the general idea of the cyclone for all space and time, 

 but not the sources of its power. These are to be found in the less 

 specific gravity, potential or actual, of the lower atmospheric strata as 

 compared with that of the higher. Air, and gases generally, expand 

 when heated, and become specifically lighter. It thus tends to rise 

 above the superincumbent colder air. (We see this illustrated in our 

 chimneys every day.) If the aii' over the surface of a plain becomes 

 heated by contact with it, its specific levity is increased, and it tends 

 to rise. But the density, and therefore weight of air, the temperature 

 and humidity being the same, is inversely as the pressure upon it. 

 Consequently, so long as the diminution of specific gravity caused by 

 increased temperature is balanced in the strata above by the diminu- 

 tion of pressure due to elevation, the heated air cannot ascend very 

 fast. If it had a chimney to rush up, the case would be different ; but, 

 not having one, it can only rise slowly by intermingling itself with 

 the superincumbent air. As soon, however, as the diminution of specific 

 gravity due to higher temperature is greater than that due to dimin- 

 ished pressure, the lower stratum will break itself a way through the 

 air above it, and rush up through the opening. This result may be 

 attained, and is generally readied, by favoring circumstances, long 

 before equilibrium is totally destroyed. When once the' heated air 

 has thus at a particular point formed itself a channel of escape, the 



