SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 407 



diminution of temperature depends therefore on the specific heat of 

 the gaseous envelope and the magnitude of the force of gravitation on 

 the surface of the body in question.' 



The equilibrium of heat that obtains in the atmosphere under such 

 a distribution of temperature will best be especially understood by the 

 consequent deduction, that any mass of air transferred from one altitude 

 to another will, in consequence of the compression or the expansion that 

 it experiences, there attain exactly the temperature of the surrounding 

 region, and will therefore have no tendency to leave its new location. 

 This is a state of indifferent equilibrium. If the diminution of tempera- 

 ture were less than 1° C. per 100 meters of ascent, then would the equi- 

 librium be '' stable"; were it greater than 1° per 100 meters, then would 

 the equilibrium of the superposed strata be " unstable." 



If now we let the sun act as the source of heat for our atmosphere, 

 and assume that the latter is perfectly diathermanous, so that it absorbs 

 absolutely no heat-rays, and therefore receives the solar heat only through 

 rhe medium of the heated surface of the earth, the stratum of air 

 directly resting upon the earth becomes heated by conduction, specifi- 

 cally lighter, and ascends, a process that is continually renewed. Since 

 now, in ascending masses of air, a cooling must take place at the rate of 

 1° 0. per 100 meters, therefore the already existing law of the diminu- 

 tion of temperature with altitude cannot be altered by this method of 

 warming from beneath. The assumed premises are, however, only in 

 part fulfilled in the actual atmosphere, and its heating takes place by 

 no means in such a simple manner. First, the atmosphere is by no means 

 perfectly diathermanous, but directly absorbs about one-fourth of the 

 solar heat-rays. Moreover, for the higher strata an important source of 

 heat is the aqueous vapor, which, carried up with the ascending air or 

 cooled from any other causes, condenses and gives its latent heat up to 

 the air, and in this way diminishes its cooling. The table on page 29 

 shows to what a great extent the cooling of moist ascending air is 

 retarded. A further [third] source of heat is the radiation from the 

 heated surface of the earth, a considerable portion of which may be 

 absorbed in the lovier strata. Inversely, the surface of the earth cooled 

 by radiation of heat at night and in winter has a cooling influence on 

 the lowest strata of air. All these circumstances bring it about that 

 observations give a diminution of temperature with altitude varying 

 according to place and time, and which cannot be expressed by any 

 law. Observations agree only in showing that on the average in the 

 lowest strata, which alone are accessible to us, the temperature diminu- 

 tion is considerably slower than 1° C. per 100 meters, and that the annual 

 average in the tropics, as in Europe, lies between 0o.5 and O^.G 0. The 

 few observations made by Glaisher above 20,000 feet give a still slower 

 temperature diminution at that altitude. 



A temperature diminution as rapid as ascending currents of dry air 

 must show has only been found in summer time during fine weather. 



