Transactions and proceedings and report of the Philosophical Society of Adelaide, South Australia . rrent—thewhirlwind—it is directly upwards, but in a more or less oblique direction,on account of the forward motion of the whole and the friction with thesurrounding air. Generally the gyrations of the whirlwind cease as suddenly as theybegin—first in the centre, then at the circumference. A dust column,seen at a distance, appears invariably to break off at some distance abovethe ground, the objects comprising the lower part rapidly descending,while those in the upper portion seem slowly to floa
Transactions and proceedings and report of the Philosophical Society of Adelaide, South Australia . rrent—thewhirlwind—it is directly upwards, but in a more or less oblique direction,on account of the forward motion of the whole and the friction with thesurrounding air. Generally the gyrations of the whirlwind cease as suddenly as theybegin—first in the centre, then at the circumference. A dust column,seen at a distance, appears invariably to break off at some distance abovethe ground, the objects comprising the lower part rapidly descending,while those in the upper portion seem slowly to float upwards, con-spicuous articles of small weight being often carried to considerable dis-tances ; the light dust preserves its columnar form sometimes for aquarter of an hour or more without apparently changing its place. In conclusion, the following is a short resume of what is intendedto be shown in the foregoing, viz :— 1. Whirlwinds are upward currents of heated air, endowed witha spiral motion at the circumference, progressive motion and smalldiameter in proportion to their 103 2. They only occur during1 the warmest part of the year andday, and in calm weather. 8. They are due to the unstable equilibrium of the air, the hotlower portion acquiring a high state of tension by being unable torise according to the laws of heat, being prevented by some rapidlymoving current of cold air. 4. They are produced by the hot air obtaining exit through anarrow vent forced through the upper current. 5. Their locomotion is produced by the vent and its cross aircurrent being carried along with the cold current. 6. Their spiral motion is produced by the shifting of the centre ofattraction (the vortex) of the hot air particles and their infringementupon each other, and the unaffected volume of the adjacent air, assistedby friction. In fact they are produced by the same causes, only inversely,as the rotating motions of fluids escaping through a narrow vent belowthem. 7. They
Size: 1216px × 2054px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1870, bookidtransa, booksubjectscience
