8 



Hon. Ralph Abercromby. Relation between 



her, the whole of the changes in the height of her barometer would 

 be due to the motion of the cyclone. If the cyclone were both motion- 

 less and of constant depth, while she moved, the barometric changes 

 would be due to her passage from a place of higher to a place of lower 

 pressure, or vice versa. In practice, the ship and cyclone are always 

 both moving, and as an additional complication the cyclone is often 

 increasing or decreasing rapidly in depth. Hence the difficulty of 

 dealing with observations near the centre of a small cyclone. Suppose 

 a ship was lying- to a short distance in front of the central point of the 

 cyclone, but that the depression was filling up very rapidly. Her 

 barometer would begin to rise before the centre passed over her, 

 while the wind would not change till the vortex had passed. Then it 

 would probably be reported that the centre of wind rotation was 

 not coincident with her barometric minimum. The centre of wind 

 rotation would, however, have been always really coincident with 

 the centre of the isobars at any particular moment. The whole 

 question deserves further attention. It should be noted that the blue 

 centre is also sometimes reported as non-coincident with the baro- 

 metric minimum. 



There are very few observations on the motion of the upper clouds, 

 but so far as they go, the general circulation of the air in a Bengal 

 cyclone appears to be identical with that in higher latitudes. 



There are no sufficiently detailed observations to enable us to con- 

 struct synoptic charts of the distribution of rain, cloud, &c, round a 

 cyclone ; but by generalising the sequence of weather in many such 

 depressions, we find squalls, showers, and dirty-looking clouds all 

 round the cyclone, with more violent squalls and torrential rain sur- 

 rounding the centre. The actual centre is always calm ; and though 

 blue sky does not seem to have been always observed, a cessation of 

 rain is usually reported. 



This clear "bull's-eye" is the most characteristic feature of a 

 tropical cyclone. Otherwise the only difference is that the rear of 

 the disturbance is not so clear in Bengal as in temperate cyclones ; 

 and that squalls are formed all round the centre instead of only in the 

 right rear of the depression. 



The sequence of weather to any observer appears to be as follows : — 

 The first indication is always a strange coloration of the sky at sun- 

 rise and sunset. The author has examined this point carefully, and 

 discovered that abnormal colours are developed not only before the 

 barometer has begun to' fall at the place of observation, but before 

 any appreciable depression can be found anywhere on the synoptic 

 charts. He has observed the same in England, and the fact is impor- 

 tant, as it proves that the examination of synoptic charts can never 

 supersede the necessity of observations on the appearance of the 

 sky. 



