CIRCULATION AND CONVECTION IN GASES. 55 



at A to A', and a fall at B to B'. The surface, however, thus sloping, 

 could not keep in equilibrium, and at once a surface-current would start 

 from A towards B. The pressure at the bottom at A would, therefore, 

 fall, while that at B would rise through 

 the removal of water from A to B. A 

 reverse current would then be started , 

 along the bottom, the excess of pressure 

 urging the cold water from B towards A. FIG. 43. 



In apparent confirmation of this ex- 

 planation, we know that in the Atlantic the Gulf Stream flows along the 

 surface from the tropics to the polar regions, while it is found, by deep 

 ocean soundings, that the temperature near the bottom, even in the 

 tropics, is not much above the freezing-point, doubtless through a return 

 under-current from the polar regions. But in all probability the surface- 

 current is almost entirely due to winds along the surface. A persistent 

 wind blowing along the surface in one direction will give far more 

 kinetic energy to the water than can be acquired from the potential 

 energy due to heat and expansion. 



Circulation also takes place in lakes and ponds when cold weather 

 sets in. The surface-water cools and falls, its place being taken by the 

 warmer water pushed up from below, and so the water is turned over 

 and over as it were, successive portions of it being cooled. But at 4 

 the process stops, for at that point the maximum density is attained. 

 When the first mass of water is cooled down to that temperature, it goes 

 to the bottom, and remains there, and the subsequent circulation stops 

 short of this layer of densest water. The non-circulating water gradu- 

 ally increases, the circulating part becoming shallower and shallower, 

 till all is at 4. Then circulation entirely ceases, and the top layer goes 

 on cooling towards 0. Hence the existence of a point of maximum 

 density hastens the arrival of the freezing-point by cutting short the 

 process of circulation. 



Convection in Gases. Convection is even more marked in gases than 

 in liquids, partly through their greater expansion with change of tempera- 

 ture, and partly through their smaller frictional resistance to motion. 



A very common instance of convection in the atmosphere occurs over 

 the heated surface of the ground on a hot summer day. The air im- 

 mediately over the surface expands and rises, owing to its diminished 

 density, its place being supplied by downward currents of cooler air from 

 above. The existence of these currents is shown by the tremulous motion 

 of distant objects looked at through the strata of air near the ground. 

 The variations in density produce refraction, and the refraction of the 

 rays is continually altered by the rapid change in position of the up and 

 down currents. 



We have similar effects in the tremulous motion of objects looked at 

 through the ascending currents over a gas-flame, and in the flickering 

 shadow of a flame thrown by sunlight. 



A special instance of these local convection-currents is afforded by 

 the haze so common on some hot days in summer. If any distant object 

 is looked at through a good telescope on such a hazy day, its outline is 

 found to be continually undulating, owing to the varying refraction. Since 

 variation in refraction is always accompanied by reflection, a considerable 



