366 C. AUbe — Atmospheric Radiation of 



Bernard and Siintis, give a coefficient about 15 per cent 

 smaller. 



(6.) It is evident from Tyndall's observations and those of 

 Angstrom and others that this coefficient must vary with the 

 chemical nature of the gases contained in ordinary air and, 

 although it is therefore of a very indefinite nature, yet we need 

 to know its probable limiting values in order to understand 

 the extent of the influence of atmospheric radiation upon 

 meteorological phenomena. On this account I have applied 

 Maurer's formulae to other meteorological observations and 

 during the past three years have mentioned to several physi- 

 cists the desirability of the direct determination of this co- 

 efficient by ordinary laboratory methods. I am therefore much 

 pleased that Prof. Hutchins has been able to make a direct 

 experimental response. The values deduced by him are con- 

 siderably larger than those given by Maurer, as was to be ex- 

 pected, because Maurer's computation considers only the 

 average radiation during cloudy as well as during clear nights 

 and also because it assumes that the convection during windy 

 nights has not obscured the simple effect of radiation. Further- 

 more there is a possibility that the heated metallic tube of 

 Prof. Hutchins may have given off an extra quantity of dust 

 or of gaseous compounds all of which would increase the ap- 

 parent radiation. But although neither of these determinations 

 represents pure, clean, dry air and although both must at pres- 

 ent be considered only as first steps in our knowledge, yet they 

 give us invaluable indications of the possible dimensions of an 

 important fundamental constant in meteorology. 



(7.) The importance of this subject in the study of the 

 mechanics of the atmosphere may be enforced by the follow- 

 ing consideration of the progress of our knowledge. 



{a.) Espy, having reasoned his way up to the conclusion that 

 rising air cools by virtue of the work done by it in its expan- 

 sion against the surrounding pressure, also clearly stated that 

 descending air is warmed by compression (i. e. by the work 

 done upon it) whence it is able to dissipate any fog or cloud 

 that it contains and becomes perfectly clear ; inversely, cloud- 

 less skies, whether in small patches or large regions, demon- 

 strate in general the prevalence of descending air. The 

 further development of Espy's principle gave rise to what is 

 called his latent heat or condensation theory of storms, the 

 items of which are briefly as follows ; ascent is due to buoy- 

 ancy ; cooling is due to the work of expansion incident to the 

 ascent ; when cooling has brought the air to the dew point 

 and to precipitation then latent heat is liberated and the cool- 

 ing is checked but the relative buoyancy of the ascending air 

 is increased, the indraft and updraft increase and the storm 



