Baron Wrede on the Velocity of Radiant Heat. 379 



and an angle of 20° 50' with the axis of the rliombohedron. 



They were found in cavities in nodules of the carbonate of 



iron used in the Iron-works at Dowlais, near Merthyr Tydvil. 



Cambridge, Feb. 3, 1842. W. H. MiLLER. 



LVI. On the Velocity of Propagation of Radiant Heat. By 



Baron J. von Wrede *. 

 ^pHIS investigation is founded on the principle, that if the 

 A heat and light in the solar rays move with different velo- 

 cities, they must show an unequal aberration; and conse- 

 quently the himirious and thermal images of the sun in a tele- 

 scope cannot overlap each other completely, but must be se- 

 parated in a direction parallel to the ecliptic. Hence the tem- 

 peratures of the eastern and western edges ol' the sun's lu- 

 minous image (i.e. of the disc we vieno) cannot be equal. In 

 order to discover this difference, the author attached the fol- 

 lowing apparatus to the eye-end of a 10-foot telescope mounted 

 parallactically, and which apparatus he laid before the phy- 

 sical section. 



To one end of a brass tube inserted in the telescope he at- 

 tached a four-sided brass box in such a manner, that its cen- 

 tral line could, by means of a graduated circle, be placed at a 

 certain measurable inclination to the plane of the declination- 

 circle passing through the optical axis of the telescope. In 

 this box he placed a small thermopile {thermokette) composed 

 of bismuth and antimony bars, their points of verticality 

 (lothstellen) being in one straight line, and in such a manner that 

 the pile could be moved by means of a fine micrometer-screw 

 in a direction perpendicular to its length. Parallel to the 

 pile's line of verticality, and in a plane per{)endicular to the 

 axis of the telescope, he fixed a spider's thread at a distance 

 from the pile nearly equal to the sun's apparent diameter, and 

 perpendicularly to this thread he fixed another which inter- 

 sected the pile in about the middle of its length. The first 

 thread may be called the vertical, and the other the horizontal 

 one. To the back of the box a terrestrial eye-piece was 

 fastened, so placed that the cross threads took up the middle 

 of its field of view. 



Before experimenting, the apparatus was thus adjusted. 

 The horizontal thread was first made parallel to the ecliptic, 



» Wc are indebted for this article to S. M. Dracli, Esq., F.R.A.S., by 

 whom it has been translated from the German, as given in Po<'<'endo'rff"s 

 AtttifUcn, vol. liii. part 4. The orij;iiial is a provisional accounrof the in- 

 vestigation given in the Fordhandlinijer ved de ^^kandinaviskcNaluiforskercs 

 an del Mode, dcr holdlcs i KjoUn/iavn/ra 3 bis 9 Ju/i 1840. 



