101 



venting other fluids at rest or only moving slowly from entering or 

 mixing with each other. As far as fluids are concerned this is illus- 

 trated in a grand scale by the oceanic currents, and by the ease a sharp 

 stream of gas or vapour can be ejected to some distance through some 

 other without mixing. Claiming the same property for the upper 

 aerial currents (which must acquire a considerable momentum, being en- 

 tirely unimpeded in their course, and therefore offer a great resistance 

 by their inertia to any intrusion), they will act as a barrier to the as- 

 cending particles of heated air, infringing upon the swift cool current at 

 right angles or nearly so, and thus are arrested in their course and 

 detained, as illustrated in Plate 3, fig. 1. Air being a bad conductor of 

 heat the warm particles will be very slow in parting with it, especially 

 to others in rapid motion, and therefore long retain their buoyancy. A 

 certain tension, a pressure exerted by them upon the barriers would 

 therefore result and increase continually, though in a slow ratio 

 Gradually the atmosphere would grow hotter, the upward pressure 

 stronger, the area of the calm extended, and the upper current forced 

 upward in the places of maximum pressure, as shown in fig. 2, plate 3. 



In the meantime the air becomes more and more oppressive, and to 

 some extent charged with moisture. The heat becomes visible, the whole 

 lower strata of the atmosphere seem quivering, undulating, strangely dis- 

 torting distant objects. The combat of the forces is approach- 

 ing its climax ; but pressure being very unequal over any extent 

 of area on account of the difference of the heat-radiating capacities of the 

 soil, at the points of maximum pressure a small volume of hot air forces 

 itself into the swift current, and instantly is swept along. This event is 

 noted — above, probably by the formation of a thin streamer of cloud by 

 the condensation of aqueous vapour ; below, by a slight puff of wind in 

 the direction of the seat of the momentary disturbance, and therefore 

 in different directions at different localities. 



When the sun has reached an altitude of 45° — 50° the time has ar- 

 rived for decisive measures — the rupture and overpowering of the upper 

 current and the escape of the imprisoned air. The equilibrium is at last 

 attained ; the overheated air suddenly forces its way at some point 

 through the rapid upper current by means of a sufficiently large vent, 

 but keeps its exit open for a ?hort time only ; then it is closed again by 

 the onward pressure. From all sides the confined air streams up into 

 the gap or towards it, but being formed in a moving medium, it is car- 

 ried along with it bodily, according to the same law that prevented its 

 entrance, viz., its viscosity when in motion. (See fig. 3). 



