

1852.] On Dust Whirlwinds and Cyclones. 265 



Description and use of the Cyclone Compass, Plates 5 and 6. 



The Magnetic points north and south, and carries a light metal 

 disk of Palladium, or other metal, marked with the wind points, and 

 capable of being shifted and reversed for the northern and southern 

 hemispheres ; by which arrangement, the wind points, are always pre- 

 served in their respective positions. The disk is also grooved, for the 

 purpose of being adjusted to the magnetic declination. 



The transparent disk placed below this, with a metal rim, represents 

 the body or zone of the Cyclone, and is marked with dotted radii or 

 with thin wires, corresponding to the wind points, which also indicate 

 the ship's place and the bearing of the centre ; all which is understood 

 by simply noticing the direction of the wind blowing at the time. 



For instance, in a storm in the Northern Hemisphere with the wind 

 at South-East, the bearing of the centre will be seen at a glance, to 

 the South-West ; with the wind at South, the bearing of the centre 

 will be West. 



For the Southern Hemisphere with the wind at South-East, the 

 bearing of the centre will be North-East ; and with the wind at South, 

 the bearing of the centre will be East. 



The transparent disk is fixed to a small cylinder, round which a 

 piece of thread is wound from right to left, if required for the 

 Northern Hemisphere ; and from left to right, for the Southern. 



The rim carries a pencil, or a pointed glass tube for ink, when 

 required to mark a course on paper. 



Placing the Cyclone Compass over the ship's place dotted on a 

 Chart laid perfectly flat on a table, and then pulling very gently on 

 the thread in a supposed track, the peculiar motions of the Cyclone, 

 as I understand it, both progressive and rotatory, will be exactly imi- 

 tated, and the veering of the winds, and the direction in which the 

 sea is propelled by them in different parts of the space over which the 

 influence of the Storm extends for the time, may be satisfactorily and 

 clearly demonstrated, as in the accompanying diagrams. 



Opposite points on the rim of the transparent disk, will then be 

 found to mark on one side a gentle curve, on the other a loop. 



On the side of progression, while the Cyclone Compass sweeps a 

 gentle curve, describing a small arc of a large circle, on the opposite 

 or looped side, it will have passed over a semi-circle of small diameter. 



