Register Thermometer without any Index. 441 



ing off a small portion of the tube. The open extremity of the 

 tube d, is inserted into a portion of a larger thin glass-tube, 

 which exactly fits it, and which terminates in a hollow bulb, 

 containing a small quantity of mercury, c, Figs. 1, & 2. The 

 inner tube is carried forward until its extremity is about oppo- 

 site to that part of the outer tube where it begins to swell into 

 a bulb ; and the two tubes are then made to adhere permanent- 

 ly, by introducing a minute quantity of colourless varnish be- 

 tween them. 



The scale of this tube commences from its upper open ex- 

 tremity, and is numbered downwards 1, 2, 3, &c. but marked as 

 in Fig. 1, 10, 20, 30, &c. 



When an instrument thus formed is held upright, the glo- 

 bule of mercury in the bulb e, Fig. 1 . falls on the open extremity 

 of the tube d, as represented in Fig. 2. ; and if the bulb p be 

 now heated by the hand, the mercury will rise in the tube, and 

 unite with the globule c, with which it will remain connected as 

 long as the instrument is kept in the upright position. If the 

 instrument be now exposed in its upright position to the air, 

 which has, let it be supposed, the temperature of 60, the upper 

 extremity of the mercury in the tube , will be opposite that de- 

 gree of the scale ; but the mercury in the tube d, will still re- 

 main at the beginning of its scale, and continuous with the glo- 

 bule c. Let the instrument now be placed in a horizontal posi- 

 tion, and the entire globule of mercury will instantly quit the 

 open extremity of d, leaving the tube exactly filled with that 

 fluid, and the globule will then take the position c, Fig. 1. when 

 the instrument rests on the edge of its scale. If, from the in- 

 stant the globule is thus made to quit the open extremity of the 

 tube d, both of the bulbs n, and p, be kept moist with a rapidly 

 evaporating fluid, such as ether, alcohol, &c. the mercury in both 

 tubes will descend equally, and will remain permanently below 

 the elevation due to the temperature of the air, as long as the 

 * 3 R3 



