88 Mr. J. Croll on Ocean-currents in relation to 



from the warm water poured into the Atlantic by the Gulf- 

 stream. 



How, then, are these winds heated by the warm water? The 

 air is heated in two ways, viz. by direct radiation from the water, 

 f>nd by contact with the water. Now, if the Gulf-stream con- 

 tinued a narrow and deep current during its entire course similar 

 to what it is at the Straits of Florida, it could have little or no 

 opportunity of communicating its heat to the air either by radia- 

 tion or by contact. If the stream was only about 40 or 50 

 miles in breadth, the aerial particles in their passage across it 

 would not be in contact with the warm water more than an hour 

 or two. Also the number of the particles in contact with the water, 

 owing to the narrowness of the stream, would be small, and 

 there would therefore be little opportunity for the air becoming 

 heated by contact. The same also holds true in regard to radia- 

 tion. The more we widen the stream and increase its area, 

 the more we increase its radiating surface; and the greater the 

 radiating surface, the greater is the quantity of heat thrown off. 

 But this is not all ; the number of aerial particles heated by ra- 

 diation increases in proportion to the area of the radiating sur- 

 face; consequently the wider the area over which the waters of 

 the Gulf-stream are spread, the more effectual will the stream be 

 as a heating-agent. And, again, in order that a very wide area 

 of the Atlantic may be covered with the warm waters of the 

 stream, slowness of motion is essential. 



Mr. Findlay says that fully one-half of the Gulf-stream passes 

 eastward and southward from the banks of Newfoundland, and 

 supposes that it is only the northern portion of the current that 

 can be effectual in raising the temperature of Europe. But it 

 appears to me that it is to this south-eastern portion of the cur- 

 rent, and not to the northern, that we, in this country, are 

 chiefly indebted for our heat. The south-west winds, to which 

 we owe our heat, derive their temperature from this south-eastern 

 portion which flows away in the direction of the Azores. The 

 south-west winds which blow over the northern portion of the 

 current which flows past our island up into the arctic seas cannot 

 possibly cross this country, but will go to heat Norway and 

 Northern Europe. The northern portion of the stream, no 

 doubt, protects us from the ice of Greenland by warming the 

 north-west winds which come to us from that cold region. 



Mr. Buchan, Secretary of the Scottish Meteorological Society, 

 has shown* that in a large tract of the Atlantic between lati- 

 tudes 20° and 40° N., the mean pressure of the atmosphere is 

 greater than in anj r other place on the globe. To the west of 

 Madeira, between longitude 10° and 40° W., the mean annual 

 * Trans. Roy Soc. of Edinb. vol. xxv. part 2. 



