313 



Above the mercury there is a small quantity of spirits of wine, 

 which extends some distance into the horizontal tube. The quantity 

 of this, and the graduation, correspond in such a manner, that the 

 extreme end of the spirit column denotes the same degree of tem- 

 perature as the mercury. The remainder of the horizontal tube is 

 filled with air. There are two moveable indices in the spirit column, 

 one in the vertical tube, the other in the horizontal, each about half 

 an inch long. The former, B, consists of a fine steel magnet enclosed 

 in glass. This forms the body of the index. At either extremity there 

 is a head of black glass, similar to that which occurs in the index 

 of an ordinary minimum thermometer. A fine hair is tied round 

 the neck of this index, between the body and the upper head ; and 

 it is made to hang down by the side, so that by its elastic pressure 

 against the tube, the index may be kept in its place, notwithstand- 

 ing its verticality. The index in the horizontal tube A, is in all 

 respects similar to that of an ordinary minimum thermometer. 



Let us now suppose the instrument fixed in its position, the first 

 part of the stem being vertical. 

 In order to adjust it, we must 

 first bring the vertical index 

 into contact with the upper ex- 

 tremity of the mercurial column. 

 To do this, let us take two small 

 but strong horseshoe magnets, 

 and lay the one above the other, 

 so that the poles of the one shall 

 overlap to a small extent the cor- 

 responding poles of the other. 

 Bring the magnets up to the 

 index in such a manner, that, 

 while the poles of the one bear 

 against the side of the glass 

 tube, the overlapping poles shall 

 lie over the tube so as to be in front of the index : the index will 

 now follow the motion of the magnets, and it may thus be brought 

 down to the surface of the mercury. In order to bring the horizontal 

 index to the extremity of the spirit column, all that is necessary is to 

 incline the horizontal tube a little downwards by pressing on the end. 



. 



