﻿872 Mr. E. A. Milne on Radiative Equilibrium : 



purpose, namely as an indicator for precise measurement. 

 A compound lever with a magnification of about 600 was 

 arranged to be moved by a micrometer at the one end, and 

 carried the contact at the other end. Using this to measure 

 the thickness of a parallel slip-gauge with the lower face 

 resting on three steel balls and the contact on the upper face, 

 it was found that repetition of observations could easily 

 be obtained with variations not exceeding 0*5 x 10~ 6 inch. 

 These experiments indicate that the advantages obtained by 

 making contact in the grid circuit of a valve may eliminate 

 some of the difficulties hitherto experienced in this method 

 of measurement. 



This method appears to have advantages in connexion with 

 the reception of feeble wireless signals w ith the aid of a relay ; 

 and it is also proposed to try it, on account of its freedom 

 from sparking at the grid-circuit contact, in connexion with 

 the location of the height of the mercury surface in the 

 vacuum space of a standard barometer. 



July 1922. 



LXXIY. Radiative Equilibrium: the Insolation of an Atmo- 

 sphere. By E. A. Milne, M.A., Fellow of Trinity 

 College, Cambridge *. 



§ 1. Introduction. — The generally accepted theory of the 

 existence of the earth's stratosphere was put forward in 1908 

 by Goldf. Gold showed that when radiation processes were 

 taken into account the continued existence of an adiabatic 

 gradient to indefinitely great heights was impossible; for the 

 upper portions of such an atmosphere, being very cold, would 

 radiate very little, but on the other hand, being backed by an 

 extensive cushion of warmer air besides the warm surface of 

 the earth, would be subjected to low-tem. erature radiation 

 of considerable intensity, and the consequent excess of absorp- 

 tion over emission would raise their temperature and so disturb 

 the adiabatic gradient. Such upper portions, however, could 

 not exchange heat with the rest of the atmosphere by con- 

 vection, for they would tend to rise, not fall. Consequently 



* Communicated by the Author. 



t "The Isothermal Layer of the Atmosphere and Atmospheric 

 Radiation," Proc. Roy. Soc. 82 A. p. 43 (1909). A preliminary 

 announcement was made at the British Association meeting in 1908; 

 see ' Nature,' vol. lxxviii. p. 551 (1908). See also Geophvsical 

 Memoirs, No. 5 (Met. Office), vol. i. p. 65 (1918). 



