On the Diathwmancy of Dry and Moist Air. 



21 



the locus of S ; , is a curve similar to and similarly situate with 

 the pedal curve the locus of N, but of twice the linear magnitude 



of the pedal curve. Or, what is the same thing, if instead of the 

 given curve we consider a similar and similarly situated curve of 

 twice the linear magnitude (the point S being the centre of simi- 

 litude), then the epicycloid the locus of S' is the pedal curve of 

 the substituted curve in relation to the point S. It may be 

 added that, in accordance with a theorem of Dandelin's, if rays 

 proceeding from the point S are reflected at the given curve, 

 then the epicycloid or pedal in question is the secondary caustic, 

 or an orthogonal trajectory of the reflected rays. 



2 Stone Buildings, W.C., 

 June 3, 1863. 



IV. On the Diathermancy of Dry and Moist Air. 

 By Professor Magnus*. 



[With a Plate.] 



IT is with regret that I find myself obliged to return to a sub- 

 ject the importance of which is not sufficient to warrant a re- 

 newed treatment of it. But inasmuch as the method which I have 

 employed for determining the transmission of heat through 

 gaseous bodies has been attacked, and since it has been asserted 

 that it is incapable of yielding trustworthy results, I felt it my 

 duty to submit it to still further examination ; especially since, 

 during my stay in London last autumn, Prof. Tyndall had the 

 kindness to show me some experiments, made according to his 

 method, which were apparently at variance with my own. 



It is known that a difference exists between the results which 



* Translated from the Monatsber. der Jconigl. preuss. Akad. der Wis- 

 sensch. zuBerlin (Sitzung vom 19 Marz, 1863), with additions from Pog- 

 gendorff's Annalen, vol. exviii. p. 575. 



