70 



Royal Society : — 



strument 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 elements 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 trans- 

 mitted 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 for- 

 midable than if a reflector were used. 



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

 closed 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 pur- 

 pose of cutting off as much as possible any connexion by conduction 

 with the tube of the telescope. 



The wires connecting the pile with the galvanometer, which had 

 to be placed at some distance to preserve it from the influence of the 

 ironwork of the telescope, were covered with gutta percha, over 

 which cotton -wool was placed, and the whole wrapped round with 

 strips of brown paper. The binding-screws of the galvanometer 

 were enclosed in a small cylinder of sheet gutta percha, and filled 

 with cotton-wool. These precautions were necessary, as the ap- 

 proach of the hand to one of the binding-screws, or even the impact 

 upon it of the cooler air entering the observatory, was sufficient to 



