310 



Mr. Huggins on the Heat of the Stars. 



[Feb. 25, 



instrument was made as great as possible by a very careful adjustment from 

 time to time of the magnetic power of the needles. The extreme delicacy 

 of the instrument was found to be more permanently preserved when the 

 needles were placed at right angles to the magnetic meridian during the 

 time that the instrument was not in use. The great sensitiveness of this 

 instrument was shown by the needles turning through 90° when two 

 pieces of wire of different kinds of copper were held between the finger and 

 thumb. For the stars, the images of which in the telescope are points of 

 light, the thermopiles consisted of one or of two pairs of elements ; a large 

 pile, containing twenty-four pairs of elements, was also used for the moon. 

 A few of the later observations were made with a pile of which the ele- 

 ments consist of alloys of bismuth and antimony. 



The thermopile was attached to a refractor of eight inches aperture. I 

 considered that though some of the heat-rays would not be transmitted by 

 the glass, yet the more uniform temperature of the air within the telescope, 

 and some other circumstances, would make the difficulty of preserving the 

 pile from extraneous influences less formidable than if a reflector were 

 used. 



The pile a was placed within a tube of cardboard, b ; this was enclosed 

 in a much larger tube formed of sheets of brown paper pasted over each 

 other, c. The space between the two tubes was filled with cotton-wool. 

 At about 5 inches in front of the surface of the pile, a glass plate (e) was 

 placed for the purpose of intercepting any heat that might be radiated from 

 the inside of the telescope. This glass plate was protected by a double tube 

 of cardboard, the inner one of which (d) was about half an inch in diameter. 

 The back of the pile was protected in a similar way by a glass plate (^). 

 The small inner tube (h) beyond the plate was kept plugged with cotton- 

 wool ; this plug was removed when it was required to warm the back of the 

 pile, which was done by allowing the heat radiated from a candle-flame to 

 pass through the tube to the pile. The apparatus was kept at a distance 

 of about 2 inches from the brass tube by which it was attached to the 

 telescope by three pieces of wood (i), for the purpose of cutting off as 

 much as possible any connexion by conduction with the tube of the tele- 

 scope. 



The wires connecting the pile with the galvanometer, which had to be 



