PAPER BY PROF. HELMHOLTZ. 89 



IV. GRADUAL VARIATIONS OF THE EQUILIBRIUM BY FRICTION AND 



HEATING. 



It is well known how very differently the propagation of changes of 

 temperature in the air goes on according as heat is added or withdrawn 

 below or above. 



If the lower side of a. stratum of air is warmed, as occurs at the sur- 

 face of the earth, by action of the solar rays, then the heated stratum 

 of air seeks to rise. This is effected very soon all over the surface 

 in small tremulous and flickering streams such as we see over any plane 

 surface strongly heated by the sun ; but soon these smaller streams 

 collect into larger ones when the locality affords opportunity, especially 

 on the side of a hill. The propagation of heat goes on relatively rapidly 

 through the whole thickness of the atmospheric layer, and when it has 

 a uniform quantity of heat throughout its whole depth and is therefore 

 in adiabatic equilibrium then also the newly added air seeks de nova 

 to distribute itself through the entire depth. 



The same process occurs with like rapidity when the upper side of a 

 stratum of air is cooled. 



On the other hand, when the upper side is warmed and the lower side 

 cooled such convective movements do not occur. The conduction of 

 heat operates very slowly in large dimensions, as I have already ex- 

 plained above. Radiation can only make itself felt to any considerable 

 extent for those classes of rays "that are strongly absorbed. On the 

 other hand, experiments on the radiation from ice and observations of 

 nocturnal frosts show that most rays of even such low temperatures 

 can pass through thick layers of clear atmosphere without material 

 absorption. 



Therefore a cold stratum of air can lie for a long time on the earth, 

 or equally a warm stratum remain at an altitude, without changing its 

 temperature otherwise than very slowly. 



Similar differences exist also in the case of the change of veloc- 

 ity by friction. For the normal inclination of an atmospheric stratum 

 its upper end is nearer to the earth's axis than its lower end. If the 

 stratum appears at the earth's surface as a west wind, then the moment 

 of rotation of the lowest layer is delayed [by resistance of the earth's 

 surface], its centrifugal force is diminished, and on the polar side of the 

 stratum this lowest portion will slide outwards, approaching the axis 

 in order to find its position of stable equilibrium at the upper end of 

 the stratum. This movement will ordinarily take place in small trem- 

 ulous streams similar to the ascent of warm air and must diminish the 

 moment of rotation of the whole layer rather uniformly, but in the 

 upper portions a little later than in the lower. Since, however, this 

 latter effect distributes itself throughout the whole mass of air, it will 

 become much less apparent on the lower side of the stratum than if it 

 were confined to the lower stratum. 



