Heat and its Importance in Meteorology. 365 



(4.) The fact that some such process is going on in the 

 atmosphere has, 1 presume, been recognized by many although 

 the published expressions in regard to it are generally very 

 indefinite. It is almost invariably the case that the cooling of 

 the air at night is attributed to the radiation of heat from the 

 earth and the fact that the surface of the soil becomes cold is 

 quoted as an evidence of this action. But it can easily be 

 shown numerically that the nocturnal conduction of heat 

 between the surface of the earth and the air above it can only 

 affect a very thin layer of air during the twelve hours of night- 

 time (3 meters according to Maurer) and that there must be an 

 appreciable radiation from the air itself. 



(5.) On this whole subject the writings of Dr. Julius Maurer 

 offer us some valuable formulae and data ; especially has he 

 been the first (see Schweiz, Met. Beob., 1885, vol. xxii, Ap- 

 pend, v, Zurich, 1887,) to give us an approximate value of the 

 coefficient of radiation of a cubic centimeter of air. This co- 

 efficient he deduces from the ordinary meteorological observa- 

 tions of the temperature of the air at nighttime as given by 

 the ordinary screened thermometers. Maurer finds that the 

 cooling of the free air during night in the lower layers of the 

 atmosphere and on the average of the whole year, namely with- 

 out distinguishing between cloudy and clear weather, or calms 

 and winds, proceeds as though a cubic centimeter of air gives 

 up to an enclosure, 1° C. cooler than itself, 0-0000007 gram- 

 calories per minute. !Now the specific heat of air under con- 

 stant pressure is 0*2377 and the density of the air to which 

 Maurer's coefficient pertains is about 0-00129 or T |-g of water. 

 We may therefore convert his coefficient of radiation in 

 absolute calories into a coefficient of cooling: in Centigrade 

 degrees whence it results that the cubic centimeter of air 

 within such an enclosure cools by radiation at the rate of 

 0-0000007X 773^-0-2377=0-0023 degrees C. per minute or 

 - 138 C. per hour. 



Maurer finds indications that this coefficient is larger in 

 summer and smaller in winter. The above average value 

 results from observations of temperature for many years at 

 St. Petersburg, Prague, Berne, Toronto and Barnaul and' may 

 be considered as belonging to air that has an average pressure 

 of 750 mm , a temperature of 5° C, is nearly saturated with 

 moisture, and contains the average amount of dust and aque- 

 ous particles that belong to the air of those stations. Maurer 

 especially notes that the coefficient thus found is larger than 

 belongs to pure, dry air on account of the solid and liquid 

 particles contained in the ordinary atmosphere and participa- 

 ting in the radiation. He further states that similar computa- 

 tions based on observations made at the high stations, St. 



