406 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



rent of air arrives at a ^iven altitude, and this is the usual question in 

 meteorological investigations, then we cannot use the above equation, 

 because it already assumes a knowledge of the final temperature 5 on 

 the other hand, our table gives at once abundantly sufficient informa- 

 tion. The initial temperature of the current of air may, for instance, be 

 10° C, and the altitude to which the air must rise = 8,000 Paris feet 

 = 2,C00 meters. The table first gives approximately the cooling = Oo.Oi 

 X 2G = 14.0, therefore a final temperature of —4P. At the altitude of 

 2,C00 meters for —4°, the temperature diminution is 0.61. The mean 

 temperature diminution is therefore 0.57 per 100 meters, and at 8,000 

 Paris feet, or 2,G00 meters, the temperature is — 40.8 C. The error 

 that we introduce would be quite unimportant if for such slight 

 altitudes we only used the initial rate of diminution of temperature. 

 While a moist current of air cools by only I40.8 C, a dry current would 

 cool 26°, and therefore show a temperature of —16° O. If now the 

 air sinks on the other side to its original level, it is warmed by 26°, 

 whether dry or moist. The temperature of the dry current of air is 

 therefore the same on both sides of the mountain, that of the moist cur- 

 rent, however, 21o.2 C, that is to say, by more than 10° C. higher on 

 the lee side. Since the air is only saturated with moisture at — 4°.8 C, 

 it must possess a great relative dryness. Thus, as is well known, is 

 explained the warmth and dryness of the Fohn. 



We will now append some general conclusions to the derivation of 

 the law according to which a mass of air changes its temperature when 

 the pressure acting upon it, and therefore its volume experience a change. 



For a moment imagine the attraction of the earth upon its atmos- 

 phere to cease, and the latter to be a uniformly dense gaseous envelope, 

 having a constant temperature at all distances from the earth's surface. 

 This gaseous envelope contains no aqueous vapor, and the influence of 

 every cosmic or telluric source of heat is excluded; then let the force of 

 gravity come into existence: the lower strata of air are, by the weight of 

 the upper, compressed together, and the density of the strata diminishes 

 with the altitude, and, according to Mariotte's law, in a geometrical pro- 

 gression. The temperature also can now no more remain the same at all 

 distances from the earth's surface, but must be highest in the lowest 

 strata, which are strongest compressed, and must diminish upward. Thcr 

 law of this upward diminution of temperature is expressed by the 

 equation of Poisson, above given, which presents the relation between 

 the temperature and the pressure of the gas to which no heat is com- 

 municated from the exterior, and from which no heat is abstracted, and 

 we have shown, page 25, that the amount of this diminution of tempera- 

 ture is 1° 0. for 100 meters difference of altitude. In the gaseous enve- 

 lopes of other celestial bodies, a diminution of temperature outward 

 must take place in a similar manner, which, so ftir as it is a function of 

 the gravitation toward the center of the mass, can be computed accord- 

 ing to one of the formulae given on pages 23 and 24. The amount of this 



