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Plate VI., which is derived from a diagram
exhibited by the author in a lecture delivered before the Meteorological Society in 1878, represents the cloud distribution in a typical circular progressive cyclone. The large arrow represents the general direction of progression, and the smaller arrows show the direction of the upper currents. We have already learnt that the approach of such a cyclone is generally heralded by " outriders," consisting of wisps of Cirro-filum moving rapidly in the outflowing upper currents to the front and the right-front of the disturbance. These wisps may be more than one hundred miles in front of, but generally are closely followed by a sheet of Cirro- velum, which spreads over the front of the disturbance, and which is usually attached to clouds of Interfret and clouds of Inversion forming the great composite bank of cloud called Nimbus (§ 85), from which we generally get extensive precipitation. These clouds will, of course, appear to be moving in different directions, according to the different altitudes at which they exist, and to the position of the observer with regard to the disturbance.