IN NATURE, vol. xii. p. 98, you notice a paper by Dr. Hann on two rival theories of cyclones. According to one, “whirl-winds are formed mechanically by different streams of air meeting, and centrifugal force causes the central depression. The more modern theory regards a local depression as the first condition, causing an indraft resulting in a whirlwind through the earth's rotation: the primary depression is held to follow condensation of vapour.”