120 Prof. TyndalFs Remarks on the Paper 6/* Prof. Magnus. 



vapour, in the experiments of Professor Magnus, does not, there- 

 fore, surprise me. 



I have at present no means of judging of the validity of the 

 assumption by which Professor Magnus accounts for the effect 

 observed when his air, instead of being passed, through cold 

 water, was urged through water at a temperature of 60° or 

 80° C. He here assumes precipitation, though there is none 

 visible. By a similar assumption he explains the experiment of 

 Professor Frankland, in which aqueous vapour was discharged 

 along the axis of a cylinder of hot air and carbonic acid, and 

 protected from precipitation by its gaseous envelope. 



With regard to the] formation of dew, the amount deposited 

 depends on the quantity of vapour present in the air ; and where 

 that quantity is great, a small lowering of temperature will cause 

 copious precipitation. Supposing 50, or even 70 per cent, of 

 the terrestrial radiation to be absorbed by the aqueous vapour of 

 the air, the uncompensated loss of the remaining 30 would still 

 produce dew, and produce it copiously where the vapour is abun- 

 dant. Attenuated as aqueous vapour is, it takes a good length 

 of it to effect large absorption. I have already risked the opi- 

 nion that at least 10 per cent, of the earth's radiation is inter- 

 cepted within 30 feet of the earth's surface; but there is nothing 

 in this opinion incompatible with the observed formation of dew. 

 A surface circumstanced like that of the earth, and capable of 

 sending unabsorbed 80 or 90 per cent, of its emission to a dis- 

 tance of 10 feet from itself, must of necessity become chilled, 

 and must, if aqueous vapour in sufficient quantity be at hand, 

 produce precipitation. 



I should willingly leave to others the further development of 

 this question, feeling assured that, once fairly recognized by field 

 meteorologists, the evidence in favour of the action of aqueous 

 vapour on solar and terrestrial radiation will soon be over- 

 whelming. An exceedingly important instalment of this evi- 

 dence was furnished by Lieut.- Colonel Strachey in the last 

 Number of the Philosophical Magazine. It is especially grati- 

 fying to me to find my views substantiated by so excellent an 

 observer and so philosophical a reasoner. 



Let me say, in conclusion, that nothing less than a conviction 

 based on years of varied labour and concentrated attention, 

 could induce me to dissent, as I am forced to do, from so excel- 

 lent a worker as Professor Magnus. Hitherto, however, our 

 differences have only led to the shedding of light upon the sub- 

 ject ; and as long as this is the result, such differences are not 

 to be deprecated. 



Royal Institution, July 2, 1866. 



