BO 4r Intelligence and Miscellaneous Articles. 



served. I expect it will enable me to determine the magnetic eqid- 

 valent-of heat. — Gornptes Rendus de VAcad. des Sciences, March 23, 

 1874, vol. lxxviii. pp. 845-847. 



EXPERIMENTAL RESEARCHES LEADING TO A DETERMINATION OF 

 THE TEMPERATURE Or THE SUN. BY FATHER A. SECCHI. 



During the last summer I made some experiments for the pur- 

 pose of determining the ratio between the solar radiation and that 

 of the electric light, in order to solve, if possible, the question of 

 the temperature of the sun. I chose this source of light as that 

 which is the least remote in intensity from that of the sun, so as 

 to lessen the divergence of opinion to which we have been conducted 

 concerning the law of radiation, according as the theory of New- 

 ton or that of Dulong and Petit has been adopted. 



To estimate the two radiations I have employed the same appa- 

 ratus, the therinoheliometer described in my work Le Soleil.' This 

 instrument, notwithstanding the objections which have been made 

 to it, appears to me especially suitable for determining, as in the 

 present case, simple differences. If I s and l c denote the absolute 

 intensities of the radiations of the sun and the carbon points, d s and 

 6 C the excesses of temperature of the black-bulb thermometer above 

 the temperature of the vicinity in the cases of the solar and the 

 electric radiation respectively, and a and 8 the apparent diameters 

 of the radiant surfaces viewed from the centre of the blackened 

 thermometer, we have 



d 3 __ I s tan 2 a 



C I c tan 2 a 

 whence. 



T = T — tan2<x 

 * e c tan*S' 



In practice it is very difficult to determine the radiating surface 

 of the carbons : they are in general very brilliant at the point ; but 

 thence their incandescence diminishes rapidly, and, besides, the arc 

 which separates them has a very different radiation. We have 

 sought to determine the surface of the radiating part of the carbons 

 by comparing their dimensions with those of glass tubes placed very 

 near, and valuing the distance at which a fine platinum wire entered 

 fusion without touching them. We have thus obtained a surface 

 almost rectangular, equal to that of a circle of 1 centim. diameter. 

 The radiation from the parts outside this limit was intercepted by 

 diaphragms. The pile consisted of 50 Bunsen elements supplied 

 with fresh acids (nitric acid of 40 degrees, sulphuric acid diluted 

 with nine times its weight of water) . The elements had a diameter 

 of 12 centims., and a depth of 20 centims. The conductors, of 

 copper, were short and very stout ; the intensity of the current was 

 such that the insulating disks of a Foucault apparatus were almost 

 instantly melted, and an iron wire 1 millini. in diameter and 2*5 



