The observer who has a chance to note from an advantageous position the development of a tornado observes that in a tolerably still air, or at least an air unaffected by violent winds--generally in what is termed a "sultry" state of the atmosphere--the storm clouds in the distance begin to form a kind of funnel-shaped dependence, which gradually extends until it appears to touch the earth. As the clouds are low, this downward-growing column probably in no case is observed for the height of more than three or four thousand feet. As the funnel descends, the clouds above and about it may be seen to take on a whirling movement around the centre, and under favourable circumstances an uprush of vapours may be noted in the centre of the swaying shaft. As the whirl comes nearer, the roar of the disturbance, which at a distance is often compared to the sound made by a threshing machine or to that of distant musketry, increases in loudness until it becomes overwhelming. When a storm such as this strikes a building, it is not only likely to be razed by the force of the wind, but it may be exploded, as by the action of gunpowder fired within its walls, through the sudden expansion of the air which it contains. In the centre of the column, although it rarely has a diameter of more than a few hundred feet, the uprush is so swift that it makes a partial vacuum. The air, striving to get into the space which it is eager to occupy, is whirling about at such a rate that the centrifugal motion which it thus acquires restrains its entrance. In this way there may be, as the column rapidly moves by, a difference of pressure amounting probably to what the mercury of a barometer would indicate by four or five inches of fall. Unless the structure is small and its walls strong, its roof and sides are apt to be blown apart by this difference of pressure and the consequent expansion of the contained air. In some cases where wooden buildings have withstood this curious action the outer clapboards have been blown off by the expansion of the small amount of air contained in the interspaces between that covering and the lath and plaster within (see Fig. 9). [Illustration: Fig. 9.--Showing effect of expansion of air contained in a hollow wall during the passage of the storm.]