CIRCULATION AND CONVECTION IN GASES. 



59 



The figure on following page (Fig. 46) shows the history of a cyclone 

 on four successive days. 



It is now easy to see how a forecast may be made. A cyclone is 

 shown by the isobars to be approaching as in Fig. 46, October 26. The 

 track of the centre is either observed or guessed at, and so the subsequent 

 positions of the cyclone are foretold. Each part of the cyclone carries 

 with it its wind and weather, and so the wind and weather may be fore- 

 told for the districts passed over by a given part of the cyclone. For 

 instance, if a cyclone-centre passes over Yalentia and is moving towards 

 the north of Scotland, south-west winds may be foretold for England. If 

 the isobars are near together, the pressure-slope is steep, and it may be 

 expected that the winds will be strong. If the centre moves more towards 

 the south, say over the Midlands, rain with varying winds may be fore- 

 told there, north-easterly winds for Scotland, and south-westerly for the 



Blue 



Blue 



Windy Cirrus 



FIG. 45. Cyclone Prognostics. (Abercromby.) 



southern coasts. It will be seen from this short account that it is ex- 

 ceedingly difficult to forecast English weather with accuracy for any 

 length of time beforehand, as the stations extend so little to the west- 

 wards. A cyclone first observed to be approaching in the west of Ireland 

 may already be well on its way towards England, with all its changes of 

 weather, before its course and nature can be accurately observed. When 

 communication with ships in the Atlantic by wireless telegraphy becomes 

 general, no doubt the proportion of successful predictions, already large, 

 will be further increased. 



Convection in Chimneys and Hot- Air Heating Systems. A 



chimney depends for its successful working upon convection. Taking 

 the case of the ordinary open fireplace, when a fire is first lighted in a 

 room, the column of air in the chimney over the grate is heated and 

 expands, some of it flowing out from the top. The air column in the 

 chimney now weighs less, and the pressure at the grate, as traced down 

 through the chimney, will be less than that at the same level in the rest 

 of the room. The air of the room, therefore, moves towards the grate, 



