The conclusion I arrived at, and which has since been confirmed by further study of the question, was, that the particles of air next the surface did not always rise immediately they were heated, but that they often remained and formed a stratum of rarefied air next the surface, which was in a state of unstable equilibrium. This continued until the heated stratum was able, at some point where the ground favoured a comparatively greater accumulation of heat, to break through the overlying strata of air, and force its way upwards. An opening once made, the whole of the heated air moved towards it and was drained off, the heavier layers sinking down and pressing it out. Sir George Airey has suggested to me that the reason of the particles of air not rising as they are heated, when there is no wind blowing, may be due to their viscosity: and this suggestion is correct. That air does not always rise when heated, appears from the hot winds of Australia, which blow from the heated interior towards the cooler south, instead of rising directly upwards. Sultry, close weather, that sometimes lasts for several days, would also be impossible on the assumption that air rises as soon as it is heated. This explanation supplies us with the force that is necessary to drive the air with the great velocity with which it moves in whirlstorms. The upper, colder, and heavier air is pressing upon the heated stratum, and the greater the area over which the latter extends, the greater will be the weight pressing upon it, and the greater the violence of the whirlwind when an opening is formed for the ascent of the heated air. There is a gradual passage, from the small dust eddies, through larger whirlstorms such as that at Lough Neagh, to tornadoes and the largest cyclone; every step of the gradation might be verified by numerous examples; and if this book were a treatise on meteorology, it might be admissible to give them; but to do this would take up too much of my space, and I shall only now make some observations on the largest form of whirlstorm--the dreaded cyclone.