HutcMns and Pearson — Air Radiation. 279 



temperature excess, would be inoperative so far as our present 

 purposes are concerned. Opposite the tin tube is placed the 

 device for heating and delivering an air column. A box of wood 

 100 cm by 35 cm by 14 cm is mounted upon trunnions so as to be 

 tilted by pulling an attached string. In its vertical position, 

 the column of hot air is delivered centrally past the opening of 

 the long tube, but upon releasing the string the box tilts back 

 out of the way. Beyond the air column stands a large black- 

 ened copper cube filled with water at the room temperature, 

 and to this all temperatures are referred. 



The box is filled with coils of iron wire which are heated by 

 a current taken from the lighting circuit, and the air flowing 

 up through them is heated in turn. The temperature of the 

 hot air is given by a thermometer having a very small bulb 

 held in the stream at the height of the opening in the long- 

 tube. 



Experiment to determine the Absorption in the Long Tube for 

 Lampblack Radiation . 



As our values of air radiation were to be obtained in terms of 

 radiation from a lampblacked surface, it became necessary to 

 inquire whether the column of air in the long tube exerts any 

 appreciable absorption upon the lampblack radiation. Langley, 

 in his work on the temperature of the moon, has shown that a 

 column of air 110 meters deep absorbs about 20 per cent of 

 the rays from a blackened surface at 100° C. If the absorp- 

 tion follows Lambert's law, it would, in a column of air 215 cm 

 deep — the depth used in the following experiment — be about 

 0*5 of one per cent, and may be neglected. The object of the 

 following was to ascertain if the absorption changes with tem- 

 perature excess. 



A second blackened tank at a higher temperature than the 

 standard tank was thrust in front of the tube and the deflec- 

 tion of the radio-mircometer noted. The following table gives 

 the results. 



Table I. 

 Depth of absorbing column, 2Jf5 cm . 

 Excess Temp. Mean Den. 



62-6 

 103-5 

 163-0 

 224-0 



4-06 



6-62 



9-55 



12-79 



12-18 



20-91 



30-60 



40-90 



50-00 



51-63 

 94-17 

 145-3 

 200-3 

 252-5 



