434 Sir James Dewar [Jan. 28, 



earth temperature 10° C). They show values below unity for clear 

 skies (curve 2), and much greater than unity for bright clouds 

 (curve 3). Exposure was first made to the black dome until no 

 further displacement was occurring (about 12 minutes), the thermo- 

 scope cell being then in equilibrium with the black body at 10° C. 

 The dome was then removed to expose the cell to the zenith ; the 

 continuous displacements that resulted are therefore in excess of 

 the black readings. The flat portion of curve 3 from the Gth to the 

 8th minute resulted from a grey patch in the cloud mass. With 

 the succeeding brighter parts the irregular rise in the manometer 

 continued, and after the 16th minute was still very rapid, whereas 

 after 12 minutes' exposure to the black dome, the manometer became 

 stationary. The interposition of the hand at this moment above the 

 aperture of the instrument produced an immediate negative response 

 (backward movement of the manometer). The positive and negative 

 fluctuations in curve 2, when exposure was made to a fairly clear 

 sky, correspond respectively to bright flecks of cloud, or a clear 

 blue zenith. 



Dull or black clouds almost invariably gave values equal to the 

 earth temperature black body. On one or two occasions, however, 

 warm cloud layers were registered, as for example when a low dark 

 cloud gave a reading 1'48 times the black at 6° C, which would 

 correspond to a black temperature of 35° 0. ; this was registered on 

 both the rubber membrane and rock-salt thermoscopes on Febru- 

 ary lo, together with several low values with clear skies. Cold cloud 

 banks were not uncommon ; on one occasion a dense uniform layer 

 gave a reading corresponding to a temperature of — 7J° C, with the 

 earth temperature at 10° C. (4.45 p.m., January 22). Black fog 

 banks registered a lower temperature than the earth : for example, 

 on December 8 a ratio of 0'82 was registered, corresponding to a 

 temperature of — 4° C. The approach to the zenith of a dense 

 black cloud, from which hail fell almost immediately afterwards, 

 gave a Z/E ratio of 0'85, or a temperature a little below 2° C, the 

 earth black body being at 14*5°C. ; had it been possible to expose 

 with this cloud occupying the whole zenith, a lower value would 

 doubtless have resulted (9.30 a.m. May 4. 1920). 



Demonstration of Cloud Formations. 



The following experiments were shown during the Discourse to 

 illustrate some aspects of the texture of clouds formed by rapid 

 cooling. 



A comparison between light and heavy clouds was made by 

 pouring some 100 cc. of liquid air into a cooled cylindrical vacuum 

 vessel having a spiral draining tube sealed in below (Fig. 13). To 

 this outlet the compressed air circuit was connected, and a rapid 



