Radiant Heat by Dry and by Moist Air. 249 



the thermopile and the source of heat on the right-hand side of it. 

 This arrangement, therefore, corresponds exactly to that of Pro- 

 fessor Tyndall. As to the rest, the procedure was again the 

 same as before ; that is to say, at the further end of the tube dry 

 or moist air was introduced, and the end nearest to the thermo- 

 pile was connected with the air-pump. The readings of the 

 scale were as follows : — 



rr , ! Air in the 

 Temperature. ; tube> 



Scale-reading. 



Difference. 



o 



18 

 17 



Dry 



millims. 

 340 

 570 

 330 

 570 

 470 

 280 

 460 



millims. 



230 

 240 

 240 



190 

 180 



Moist 



Dry 



Moist 



Moist 



Dry 



idoist 





Here also a deflection towards increasing numbers was caused 

 by heating the right-hand side of the thermopile ; so that the in- 

 crease of the deflection by 230 millims., when on the left 3 feet 

 of dry air were replaced by air saturated with vapour at 18°, like- 

 wise indicated a simultaneous increase of the absorption of rays 

 of heat. 



Against all these experiments the objection might still be 

 raised that, differen tkinds of gas-strata having been interposed on 

 the two sides of the thermopile between the latter and the sources 

 of heat, a different quantity of heat might have been reflected 

 by them at their respective limits. In order to remove any such 

 influence, which would certainly be a disturbing one, the follow- 

 ing modified form of the apparatus was finally constructed. 



Third Series of Experiments. 



In the new disposition of the apparatus, two new tubes, each 

 15 centims. long, but in other respects constructed similarly to 

 the tubes 60 centims. in length already described, were intro- 

 duced on opposite sides of the thermopile, and in such a manner 

 that on the left, at a distance of 10 centims. from the funnel- 

 shaped opening, the longer tube was placed ; and then at 10 cen- 

 tims. beyond it came the shorter tube ; and lastly, at a distance 

 of 10 centims. from the last, the Leslie cube was placed. To the 

 right, on the contrary, came first the smaller and then the greater 

 tube, the intermediate distances being the same. Each of the 

 short tubes was also provided with a lateral aperture. The three 

 lateral apertures of the two tubes nearest to the thermopile, that 

 is to say, of the long one on the left and the short one on the 



