2 POPULAR LECTURES AND ADDRESSES. 



to recall attention to Dr. Wclls's admirable work. 

 In the first place, when Dr. Tyndall announces, as 

 a result of his experiments on radiant heat, that 

 " It is perfectly certain that more than ten per 

 cent, of the terrestrial radiation from the soil of 

 England is stopped within 10 feet of the surface of 

 the soil," by the absorption it suffers from aqueous 

 vapour ; it must be remarked that this absorption 

 cannot go on at the same rate through any great 

 thickiu >s of air. Eor at the same rate half the 

 radiant heat would be absorbed in 70 feet ; } in 

 140 feet ; I in 210 feet, and so on, which is incon- 

 sistent with known facts ; as, for instance, the 

 influence of clouds on terrestrial radiation. Hence 

 the quality of rays which passes through the 

 lowest 10 feet of air suffers less than ten per cent, 

 of absorption in the next 10 feet ; and it is quite 

 i in that after passing through several times 10 

 feet of air, the radiant heat must, by having been 

 deprived of the part of it specially liable to absorp- 

 tion by anurous vapour, be in a condition in which 

 not one- per cent, is absorbed from it in its passage 

 through lo feet of clear air. If true vapour of 



