[BARNES] TEMPERATURE RECORD OF NOCTURNAL RADIATION 137 



cover, was placed over the thcnnomoter P, and at the same time the 

 glass cover was slipped out. At the line c, the salt solution was poured 

 into the trough and covered the glass bottom to a depth of about 1 cm. 

 The solution had been prepared in the afternoon and allowed to stand 

 in a flask outside several hours in order that its temperature should 

 be equal to tJiat of the thermometer. At d the glass cover was put 

 over the trough, so that at this point the radiation was penetrating 

 two sheets of glass and about one centimetre of salt solution. At e the 

 trough was removed and the glass cover slipped back into place leaving 

 the conditions the same as at first. It was impossible, of course, to 

 obtain a trace through the water alone without first passing it through 

 some transparent rigid substance. Hence, this test of penetration is 

 only on such rays as succeed in passing unabsorbed through glass. We 

 can say from this test that water would not absorb more than this, 

 but at the same time it might absorb less. The effect of the radiation 

 through the water was enough to cool the thermometer nearly a degree 



.6/7 /WcrcA 06. 



Fig. 13. Radiation Through Salt Solution. Minimum Air Tempkrature 



14» Fahr. 



and a half. Unfortunatel}^, I did not obtain the maximum cooling 

 with the thermometer exposed directly to the sk}', to give a comparative 

 test for the maximum radiation at that time. The general direction 

 of the trace is to show the radiation slightly falling off after 8.30 

 o'clock quite independent of the presence of the various screens. 



I endeavoured to obtain some clear substance which would cut off 

 less of the radiation than glass. Mica is known to be fairly trans- 

 parent to the shorter heat rays, but from the character of the trace 

 shown in Fig. 14 it is evidently fairly opaque to the earth's radiation. 

 In Fig. 14, at 3.30 p.m., the cover of C was opened and the 

 maximum radiation at that time determined with no screen over the 

 thermometer. The trace is, of course, in this case over to the right. 

 As soon as the trace was obsen^ed to be steady, a thin mus- 

 covite mica plate was placed over the thermometer. The screening 

 effect was sufficient to bring the trace back to zero almost at once, 

 where it continued until about 5.30, when a small deviation to the 



