236 Mr Joseph J. Murphy on Revolving Storms. 
1° Fahr. for every 294 feet of ascent ; but the temperature of 
dry or not saturated air will diminish at the rate of 1° for every 
183 feet. Suppose that the air is nearly saturated in its lowest 
strata, and cloudless above, and that for the first few thousand 
feet of ascent the temperature diminishes at the rate due to 
convective equilibrium in dry air (conditions that sometimes 
occur in summer weather), the slighest increase of the tem- 
perature of the lowest stratum will produce an ascending 
current, in which the cold of diminished pressure will cause 
condensation to begin at a moderate height, and from that 
height upwards, the decrease of temperature will be at the 
rate of 1° Fahr. for 294 feet, while in the surrounding air it 
is at the rate of 1° Fahr. for 183 feet ; so that, at any given 
level, the air in the ascending column will be warmer and 
lighter than the surrounding air at the same level, and a 
powerful upward current will be produced, with a powerful 
indraft at its base. Of this nature, apparently, are many 
violent storms of small extent; they are usually, perhaps 
always, accompanied by heavy rain or hail from the vapour 
condensed above. 
Revolving storms or cyclones differ from these, primarily, 
in their larger extent. A cyclone cannot be formed unless 
the radius, within which the indraft due to the central baro- 
metric depression is felt, has a sensible magnitude in com- 
parison with the earth’s quarter-circumference. At its 
centre is an area of dense cloud, great barometric depression, 
and no wind. ‘The air revolves round this, and has also an 
inward motion, producing a spiral resultant. The wind is 
most violent near the calm centre, and grows less so out- 
ward, They rotate in opposite directions in the northern and 
southern hemispheres ; in each hemisphere they rotate in 
the same direction in which the earth rotates round an axis 
drawn perpendicular to its surface, as shown in Foucault's 
pendulum experiment, that is to say, against the direction of 
the hands of a watch in the northern hemisphere, and with it 
in the southern. This is caused by the earth rotating under 
the currents of air, as it does under Foucault’s pendulum, — 
and deflecting their paths on its surface to the right in the 
northern hemisphere, to the left in the southern ; thus, in the 
northern hemisphere, the current from N towards an area 
