Baron Wrede on the Velocity of Radiant Heat. 38 1 



the sun's disc was higher than that at the western edge. The 

 regularity in the quantitative determinations by this one set 

 made it extremely improbable that the observed difference of 

 temperatures at the eastern and western edges were owing to 

 errors of observation. By the other set (which consisted like- 

 wise of six pairs of observations, and whereat the telescope was 

 reversed in order to eliminate that cause of error), the results 

 were "dcithout exception in favour of the higher temperature 

 of the eastern edge, although the quantitative determination 

 did not exactly observe the same regularity that the first set 

 did. But this regularity was not to be expected, partly owing 

 to the sky being not free from small clouds, and partly by a 

 stronger breeze causing sometimes an agitation of the ap- 

 paratus. Although hitherto the requisite number of observa- 

 tions has not been made in order to solve the problem com- 

 pletely, yet the author thinks it extremely probable, from the 

 already made experiments, that heat has a greater aberration, 

 and consequently a less velocity than light. 



The average result of these two sets gives the displacement 

 of the thermal image of the sun from the luminous disc equal 

 to 0*28 turns of the screw. As each of these = 1-1 19th 

 Swedish decimal inches (ten to the foot? Tr.), and the focal 

 distance of the telescope = 101*5 of the same inches, the 



0*28 



difference of these aberrations = ; ^=4"'78, 



119x101-5 sm 1" ' ' 



whence the velocity of heat : that of light : : 20"-25 : 25"-03 

 as 4- : 5 nearly. (153,000 English miles per second. Tr.). 



l^ote by the Translator. — Although approving of the prin- 

 ciple and method of these observations, yet before assenting to 

 this numerical result, it would be advisable, I think, to test by a 

 photometer the intensity of the solar light on each side of the 

 sun's centre in the direction of a great circle of the sphere, or, 

 as in this case, in the declination-circle for that day. The 

 light will be found to be intense from the centre to the edge 

 of the disc; but if beyond this the light sensibly diminishes 

 in intensity with the elongation of the direction of the instru- 

 ment from the sun's centre, the above-found result of the 

 greater thermal intensity of the eastern edge only proves the 

 thermal image to be to tiie east of the luminous one; but they 

 may be distant some degrees apart with the same result. 

 M. Wrede has however asserted that he has found the two 

 points which possess equal temperatures: it is much to be re- 

 gretted that this otherwise valuable paper does not contain the 

 numerical measurements and the method of interpolation, 

 which would liave at once removed or confirmed the transla- 

 tor's doubts. Tiie displacement of the thermal prismatic spec- 



