in the Air and in the Sea. 29 



which may have lasted an hour, the blue water had moved along 

 the surface, and the red along the bottom ; but, notwithstanding 

 the pretty considerable difference of temperature and the length 

 of time, the coloured water particles had travelled only a few 

 feet. This experiment proves, therefore, only what we have 

 already said — -that through difference of temperature an ex- 

 change of the water particles must take place, but that this 

 exchange proceeds very slowly, even with considerable difference 

 of temperature and with little distauce between the differently 

 heated waters. 



In nature, however, the difference of temperature of sea- 

 water is proportionally inconsiderable, never amounting in the 

 whole to more than about 30° C, while this chfference is dis- 

 tributed over the vast distance of the polar from the equatorial 

 seas. It thus appears, then, impossible that this cause can have 

 power to set in motion such a current as the Gulf- stream. Even 

 the mass of the heated water, which is so readily invoked, cannot 

 here exert any accelerating action, because only an incon- 

 siderable superficial layer is warmed by the sun, and nothing 

 hinders the direct, gradual, and immediately complete inter- 

 change of the water particles. Only when large basins of 

 water of different temperatures are united by a channel can the 

 mass of the warmer water play a part, and the difference of 

 temperature generate a considerable current in the channel. 

 Thus, for example, we may regard the northern part of the 

 Atlantic between Norway and Greenland as a broad channel 

 connecting the north-polar basin with the ocean. 



The air, however, expands 15 times as much by heating as 

 water, and the influence of temperature-difference on air- 

 currents is undeniable ; yet even here that influence is generally 

 very much overrated. For air, as well as water, is a bad conductor 

 of heat, and therefore only slowly changes its temperature and 

 therewith its specific gravity. Hence we are decidedly of 

 opinion that the expansion of air by heating in the open can at 

 most occasion only a gradual inflow of air, never a sudden gust 

 or a rapid fall of the barometer. The gradually heated air 

 rises only gradually and slowly, and is just as slowly replaced 

 by cooler air. A burning light, or a chimney-fire gives us the 

 best proof of the correctness of this assertion. Although the 

 temperature produced in the fireplace is incomparably higher 

 than ever occurs in nature through the heat of the sun, and the 

 draught is artificially increased by the height and narrowness 

 of the flue, yet this draught is so inconsiderable that it can 

 seldom carry up a piece of paper thrown into the chimney, and 

 even the ashes of burnt paper are hardly lifted. This shows us 

 that even a strong heating occasions only a slow ascent of the 



