through Dry and Humid Air, 49 



" Thus far I operated on the air exactly as in the experiments 

 recorded in the foregoing letter. I then quadrupled the velocity 

 of the current through the tube, introducing in the first place 

 dry air : the needle in a first experiment moved 6 degrees in the 

 direction of cold ; but on repeating the experiment with both dry 

 and moist air no effect whatever was produced. I now removed 

 the tube and delivered a gentle current of dry air into the cone 

 of the pile, immediately the needle moved 90° in the direction of 

 cold-, the current was continued uninterruptedly for ten minutes, 

 during which time the needle gradually returned to nearly its 

 original position. The current of dry air being now stopped, 

 the needle moved 40° in the direction of heat, returning again 

 gradually and slowly to its normal position. The same tempo- 

 rary deflection for heat was also produced in an exalted degree 

 when the dry current was immediately succeeded by a moist one. 



" These supplementary experiments lead me to the following 

 conclusions : — 



T " 1st. The gentle currents of air which were caused to flow 

 through the tube in the experiments detailed in my letter did 

 not in any way disturb the results of those experiments, neither 

 would they have done so in any material degree even had their 

 velocity been quadrupled. 



" 2nd. The impact of air drier than that previously in contact 

 with the pile cools that face of the instrument with which it 

 comes in contact, whilst the like impact of moister air produces 

 the opposite effect. 



" 3rd. It is, however, impossible to confound the effects ob- 

 tained in the above experiments on transcalency with those pro- 

 duced by the impact of dry and moist air upon the face of the 

 pile, because in the first place the former are permanent, whilst 

 the latter are essentially transitory ; and in the second place the 

 deflections due to the impact of dry or moist air against the face 

 of the pile are always in the opposite direction to those obtained 

 by the interposition of the same kind of air in the path of radiant 

 heat. Thus, if the heat-rays falling upon one face of the pile be 

 made to traverse dry air, the needle will move in the direction of 

 heat, but if the apparatus be so arranged as to cause the dry air to 

 impinge upon the face of the pile, the effect due to the greater 

 transcalency of the dry air would be at first more or less neu- 

 tralized, or even altogether overborne, by the cooling influence 

 due to evaporation at the surface of the pile so brought into 

 contact with dry air. — E. F." 



" Royal Institution, 

 June 20, 1863." 



In my remarks on the experiments of Prof. Magnus, I had 

 pointed out two probable sources of error in the method which 

 Phil Mag, S. 4. Vol. 26. No, 172. July 1863. E 



