310 Correspondence with the Board of Trade [June, 



the sky will have been comparatively clear and free from cloud, while the 

 temperatures, when free from the influence of radiation, will have been 

 somewhat below the average. 



" Suppose now the northern barometers rise rapidly above the average, 

 while the temperature remains low and the sky clear. This is an indication 

 of more wind from a northern quarter, but probably without rain. Should, 

 however, the southern barometers fall at the same time, while the tem- 

 peratures in the south are much higher than in the north, the wind will 

 probably increase to a gale from a northern quarter, and a sudden chilling 

 of the atmosphere in the south will ensue, causing rain. 



"The first approach of a southerly current is usually indicated by a 

 diminution of pressure (falling barometers) in the north and west, caused 

 by a failure of the polar current ; the upper clouds are seen to be passing 

 from the south, and the temperature increases. 



" Occasionally a temporary failure of both currents takes place. We 

 may have very low barometers, but for a day or two little or no wind. 

 Such a state of things, however, never continues long. It is then espe- 

 cially necessary to watch for the first signs of approaching wind. The first 

 indication is usually an increase of pressure in the direction from which 

 the wind is coming. Should the French barometers rise rapidly or stand 

 (say) an inch higher than in Scotland, the result will be a gale from the 

 southward. Should both the French and the Scotch barometers rise rapidly, 

 while in Ireland and central England the pressure continues very low, we 

 may feel sure that both currents are approaching in force, and that the 

 collision will be violent, causing much rain, and (according to the angle of 

 incidence) either south-west and north-west gales, or a cyclonic movement, 

 which experience has shown will probably advance in an easterly or north- 

 easterly direction. On the other hand, should the increase of pressure be 

 gradual and general, the combination of the two currents will be gradual 

 also, and, though there may be rain, the winds will not be violent. 



" There are a few grand rules, which, though not always free from dis- 

 turbing causes, may be considered as generally holding good. 



" 1st. The essentially distinct characteristics of the two main currents 

 should never be forgotten. 



" 2nd. The direction of wind is usually from the place of high baro- 

 meter towards the region of low barometer. 



"3rd. The force of wind is usually proportional to the differences of 

 barometric pressure, not (as has been asserted by some) to the actual 

 pressure. It matters little how low the barometer may be, if it is equally 

 low for a considerable distance around. In such a case wind cannot follow 

 at once, for there is no available supply at hand. 



" 4th. It was believed by Admiral FitzRoy that there exists a lateral 

 transference of the whole body of atmosphere eastward. 



" Electrical and auroral occurrences should be carefully watched ; and 

 the influence of high land, &c., must be borne in rnind. 



