156 ANNUAL OF SCIENTIFIC DISCOVERT. 



ON THE POLARIZATION OF ATMOSPHERIC HEAT. 



The observations of M. Arago and Sir David Brewster have long 

 since established, that the light by which our atmosphere is illuminated 

 is polarized in certain directions. It might be supposed from analogy 

 that the heat proceeding from the same source is endowed with simi- 

 lar properties ; the following experiments place this supposition be- 

 yond the pale of doubt. 



The means of polarizing a ray of heat, without greatly diminishing 

 its intensity, are less perfectly known than those of polarizing a ray 

 of light, and the result is a corresponding inferiority in the exactitude 

 with which the calorific ray can be analysed. In the use of the thermo- 

 electric pile, the experimenter must be on his guard against numerous 

 sources of error. The blackened face of the instrument radiates into 

 space, and is cooled to a degree which depends partly upon the trans- 

 parency of the air, partly upon its temperature. The other face, 

 although protected by a closed tube, is not entirely free from the in- 

 fluence of conduction in prolonged experiments. The thermometric 

 state of the atmosphere changes capriciously every moment, owing to 

 the unequal mixture of the ascending and descending columns of air. 

 The variations in the transparency of the air, the calorific reflexions 

 -which proceed from the surface of the earth and from clouds, render, 

 in general, the intensity of the heat radiating in any given direction 

 extremely inconstant. 



These obstacles being known, I endeavored to combat them by the 

 following arrangements : The thermo-electric pile of Melloni was 

 placed in a capacious chest, so that its uncovered face was turned 

 towards an opening in the center of one of the sides. This face is 

 provided with its cone of polished brass fixed in a cylinder of wood, 

 which is lined with a tube of paste-board. The extremity of this 

 tube enters the circular opening in the side, and moves in it with 

 strong friction ; screens and diaphragms of various substances can be 

 attached to it. At its extremity, the piece destined to contain the an- 

 alyzer is fixed level. It carries a collar, to which the hand imparts a 

 rotative motion by means of a strong handle, and which carries an 

 index pointing to a dial, three decimetres in diameter, fixed against 

 the chest. This piece is surrounded by a cylindrical case of white 

 paste-board blackened in the interior, six decimetres long, open in 

 front, and destined to circumscribe the portion of space to be exam- 

 ined, the oblique rays being arrested. 



The analyser which I made use of in my first experiments was a 

 pile of thin plates of mica. It would have been easy to render it 

 movable round a line perpendicular to the axis of the thick paste- 

 board cylinder which enclosed it; but I preferred arresting it at an 

 angle of 35 with its axis, and placed a similar pile parallel to it and 

 six centimetres in advance. This assemblage polarizes and analyses 

 the heat completely; it prevents the currents of air frorn^ acting upon 

 the solders of bismuth and antimony, and destroys the radiation of 



