BUYS-BALLOT'S LAW. 115 



which show that the barometer is from two to three-tenths higher in their 

 winter than in their summer. Now it seems very probable that there is 

 not a corresponding change in the pressure over the warm water to the 

 South of Africa, which would cause an increased barometer difference, 

 and therefore, according to Buys-Ballot's Law, an increase in the force of 

 the Westerly winds. 



" The same Law probably holds good off Cape Horn ; and as we may 

 uppose that a higher pressure sometimes exists over the South Shetlands 

 md Graham's Land, than over the water to the North of them, Buys- 

 Ballot's Law would require that Easterly winds should be more common 

 in the neighbourhood of that land than near Cape Horn, as experienced 

 by Captain James Gales. 



" This intimate relation between barometer differences and the direction 

 and force of the wind increases the value of barometer observations, for it is 

 clear that when their relation is better established, we shall be able to 

 form a good estimate of the one by knowing the other, and the method 

 now adopted in the office of sifting all our data into one-degree squares 

 for each month will, it is hoped, enable us to draw monthly isobarics for 

 important parts of the sea, which will be related to the direction and 

 force of the winds prevailing in each month. Such work for a few of the 

 important squares near the Cape of Good Hope and Cape Horn will show 

 where the barometer difference is least in a given distance, and as a con- 

 sequence where the wind is generally most moderate, whilst the direction 

 in which the isobarics run will show the direction of the prevailing wind, 

 helping the navigator to decide on the best route. Seamen, who fear a 

 calm more than a gale, will be glad if we can give them the monthly 

 positions of the different areas of high and equal pressure where there are 

 calms, in order that they may avoid them." 



(35.) In his pamphlet, " Weather Forecasting for the British Islands," 

 1890, Captain Toynbee makes the following remarks regarding permanent 

 areas of high and low barometric pressure over the Ocean : — 



" Now, remembering that the force of a wind depends upon the amount 

 of difference between barometers at a given distance from each other, if 

 we can show that there are certain localities where the barometer is 

 generally high and others where it is generally low, then, if a low-barometer 

 system appears between such high and low barometer areas, the winds on 

 the side of that system which is nearest to the high barometer will be 

 stronger than those on the side which is nearest the low barometer. Such 

 a case occurs in the Northern part of the Atlantic Ocean, where, in the 

 latitude of the Azores and to the Southward of them, the barometer is 

 generally high, there being an accumulation of air on the Northern verge 

 of the N.E. Trades. The position of this area of high barometer is pretty 

 constant, although it changes its latitude with the sun, coming North in 

 summer and receding South in winter. 



" To the Southward of Iceland there is a part of the Atlantic where th« 

 barometer is generally lower than it is in the space surrounding it. This 

 is more particularly the case in winter, when there is a great difference 

 between the temperature of the warm water brought into that neighbour- 

 hood by the Gulf Stream, and that of the cold air over the neighbouring 



