Tornado is a local disturbance generally extending over a small area, but exceedingly destructive and generally accompanied by violent hailstorms, thunder, or waterspouts. Tornadoes are by no means all equally violent, but all - even the mildest - are characterised by a rotary motion, hence the name tornado. When at any time the atmosphere at any spot is in a state of instability and a stream of hot air from below forces its way through the cool layers above, we have the possibility of a tornado. The place of the rising stream is filled by air rushing in from all surrounding parts, and a vertical circulation will continue so long as the initial cause of the instability is maintained. The vertical motion may extend for several miles in height, but the horizontal effect is seldom felt over a distance of more than half a mile. The temperature of the rising current exceeds that of the air immediately surrounding it, and its pressure is proportionally less; if the cause of the disturbance reaches high altitudes, the excess of temperature of rising over the surrounding air increases, and the vertical circulation extends to a great height. There is a certain point reached by the ascending current, where its temperature has become equal to that of the air outside it; and if the vertical circulation were the only active motion, the pressure of the two portions of the atmosphere would also be equal there. Up to this point a constant impetus has been given to the ascending current, owing to its density being less than that of its neighbouring fluid. Consequently the velocity has increased, but with a diminishing acceleration. Above this level, however, the rising current becomes denser than the air around it, and therefore its upward velocity is retarded. It meets, as it were, with a resistance to its passage, and consequently often spreads out horizontally on all sides. It is seldom, however, that the air surrounding this vertical disturbance has been perfectly free from motion, and the slightest rotatory movement in the in-rushing air at the base of the column is sufficient to produce a gyratory circulation in the whole uprising stream. A spinning column of air is thus produced. The whirling of the air has an enormous effect on the pressure, and causes it to be extremely low in the centre of the tornado, while it rises rapidly towards the outside. This causes many curious effects in the passage of a tornado. The door of a closed room is suddenly confronted with the centre of the column; the pressure on the outside of the door is therefore suddenly diminished, but the air inside exerts its pressure as usual, and the result is that the door is burst open outwards. In the same way the walls of feeble houses are projected prostrate on the ground. burst and leap from their frames, the lid of a box will suddenly rise up as though gravitation had stopped. At the base of the rotating column friction with the earth's surface greatly diminishes the gyratory motion; hence centrifugal force does not make these warmer particles rush outwards sufficiently forcibly to prevent the entrance of the surrounding air. Higher up, however, friction has little or no power. The rotating particles push back the outside air, and thus form an impenetrable wall for the whirling pillar. This naturally increases the force of the inrush from below, and places enormous power in the possession of the ascending current. Pieces of timber, weighing several hundred pounds, have been lifted from the houses for a quarter of a mile, trees have been wrenched up by the roots, horses and waggons lifted into the air, houses turned about, engines overturned, and rails torn up from the ground, while even the wool and feathers of living birds were not insignificant enough to elude the vigilance of this devastating monster. Tornadoes are specially violent in the United States, and the Signal Service has published exhaustive interesting accounts of many of them, Paper No. 4 by Lieutenant Finley dealing graphically with the fearful results in Kansas, Nebraska, Missouri and Iowa caused by the memorable tornado of May 29 and 30, 1879. The reader is referred to Ferrel's Popular Treatise on the Winds for further information on this wonderful phenomenon.