434 ri:Ki<i:siKiAL i'liNsics. 



should I)c siluattxl near the trollies, so that the midday height 

 of the sun should be as great as ])ossible. in all these respects 

 South .\frica is well situated, and observations made on one of 

 the highest peaks in the Drakensberg. simultaneous'ly. if possi- 

 l)le, with similar observations ai other stations at lower levels, 

 would be of the utmost \alue. The observations should, if jjossi- 

 ble, include determinations of total radiation and also of the 

 intensities of special wave-lengths, selected to test the general 

 .absorption .and -c.atlering and also the selection a])sor])tion of the 

 atmosphere. 



2. The research just suggested involves another, which is 

 important on its own account, namely, the ([uantity of radiant 

 energy absorbed, scattered and radiated at various levels in the 

 atmosphere. This continually varying indux and efflux of 

 energy is the ])rime factor in determining the temi)erature and 

 movements of the \arious layers. It is fairly clear that the 

 lower turbulent layer, with its roughly adiabatic distribution of 

 temperature, receives its lu-at luostly from the ground by con- 

 duction. con\ection, and n-Hection. while the Ujjper, serener, iso- 

 thermal laver has its temperature maintained by the daily influx 

 ;m«l nightly efflux of radiant energy. But there is still nuich 

 to be (lone ere an\- ])recise summary can be luade of the relative 

 proportions and absolute magnitudes of the different i)rocesses at 

 work. There arc indications, too. that the division of the 

 atmosphere into two layers is not (|uite a full account of the 

 whole phenomenon. 



3. The third line of research which I wish to suggest is an 

 extensiye one, l)Ut it is also one in which it is high time thai 

 South Africa should ]jlay the ])art for which it is so obvious!;, 

 htted — I mean, the investigation of the temperature, pressure, 

 and movements of the atmos])here. and es])ecially of the upper 

 atmosp'here. by modern methods, including cajitive kites and 

 balloons and ballons-sondes fitted with measuring instruments. 

 The two circumstances that make a special call on .South African 

 physicists in this matter are the comparative simplicity of the 

 South African climate and its i)osition near latitude 30^ .S. In 

 dealing with ])henomena so complex as those presented to us 

 by the atmos])here, the advantages <if simplicity are so obviou< 

 that the point need not be laboured. As regards the im])ortance 

 of latitude 30° S.. that ap|)ears to arise essentially from the facv 

 that that ])arallel divides the surface of the .Southern Hemi--- 

 phere into two ecpial areas, so that the jiarallel appears to be the 

 main dividing line between trade winds and anti-trades as re- 

 gards horizontal mo\ement, and between the great upward 

 equatorial and the great downward i)olar trend of the air. The 

 yearly shift oif the sr.n in the sky may be regarded, however, as 

 roughly carrying the whole meteorological system with it. so 

 that we in .South Africa are well within the e(|uatorial belt in 

 summer and well within the polar l)elt in winter. We are thus 

 in a ])ositioii to ex])lore both regions with facility, and are able, 

 as it were, ti) draw a north and south \ertical section thrrniirh 



