THE ANTI-TEADES OR PASSAGE WINDS. 191 



be supposed to cause an unusually low pressure, for, other circumstances 

 being the same, it is found that the barometer is lower over warm than 

 over cold water. For instance, the meteorological data of the 10° square 

 between the Equator and 10° N., and from 20° to 30° W., show that the 

 lowest pressure exists over the hottest water (62).* 



Bermuda seems to be peculiarly placed with regard to the winds of which 

 we are speaking, for the observations given in the first number of the publi- 

 cations by this office, as well as those given by Buchan, together with others 

 sent by General Lefroy, which are quoted in this paper, show that these 

 islands are subject to alternations of Northerly and Southerly winds in 

 winter, whilst in summer scarcely any Northerly wind is recorded. 



The fact of this alternation between Northerly and Southerly winds in 

 winter seems to show that the position of this area of low pressure may be 

 variable, and that it may sometimes lie to the Eastward, sometimes to the 

 Westward of the islands. When summer sets in, and an area of low 

 pressure takes the place of the high pressure over the land, the element 

 for a Northerly wind is taken away, so that Bermuda and the American 

 coast are subject to Southerly winds. 



It will be readily understood how this alternation of high and low baro- 

 meter over the land in winter and summer accounts for the extremes of 

 cold and heat experienced by the East Coast of America, whilst we on the 

 Eastern side of the Atlantic are, in accordance with Buys-Ballot's Law, 

 subject to Southerly winds in winter, the highest barometer being then to 

 the Eastward of us. 



To return to our subjeci : If the trough of low pressure cannot be shown 

 to change its position, we have strong proof that frequent collisions take 

 place between the Northerly and Southerly winds which blow in its neigh- 

 bourhood, causing eddies, and that these eddies pass over Bermuda on a 

 North-Easterly route, producing first a South-Easterly and then a North- 

 westerly wind. We are thus led to the conclusion that these gales are 

 caused by a collision between opposing currents of air. 



The few storms dealt with in this paper support the idea that most of 

 the winter snow-storms of the Coast of America are connected with these 

 gales. Besides the trough of low pressure over the Gulf Stream to which 

 we have alluded, Buchan's charts show a larger area of depression in the 

 neighbourhood of Iceland at this season of the year, which is related to 

 the prevailing winds of winter. It is probable that this low pressure is 

 frequently intensified by the areas of low pressure which travel towards it 

 from the East Coast of America, and that these areas of low pressure, taken 

 in conjunction with the very high pressures which generally exist over the 

 land in winter, are the causes of our winter gales. 



2. We will next consider the tracks of these gales. All the evidence 

 gained by studying the data for these 11 days shows that they move to the 

 North-Eastward, and probably at a mean rate of nearly 30 miles an hour. 



To give a graphic representation of their progress, the last chart has been 



• The conclusions derived from a study of the data for this region for January will ba 

 found in the Report of the Meteorological Committee of the Royal Society for 1871. 



