Register Thermometer, -without any Index. 93 



usual manner. The other tube b, is not hermetically sealed, 

 but left open at its upper extremity c, which must be made 

 flat and smooth. This, in general, is easily effected, by mak- 

 ing a scratch with a sharp-edged file, previous to breaking off 

 a "small portion of the tube. The open extremity c of the 

 tube, 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. 2, and 3. 

 The inner tube is carried forward until its extremity is about 

 opposite to that part of the outer tube where it begins to swell 

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

 manently, by introducing a minute quantity of colourless var- 

 nish between them. 



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

 tremity c, and is numbered downwards 2, 3, 4, &c. but mark- 

 ed 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. 2, falls on the open extre- 

 mity c, of the tube b, as represented in Fig. 3 ; and if the 

 bulb n be now heated by the hand, the mercury will rise in 

 the tube, and unite with the globule c, with which it will re- 

 main connected as long as the instrument is kept in the up- 

 right position. If the instrument be now exposed in its up- 

 right position to the air, which has, let it be supposed, the 

 temperature of 60°, the upper extremity of the mercury in the 

 tube a, will be opposite that degree of the scale ; but the mer- 

 cury in the tube b will still remain at the beginning of its 

 scale, and continuous with the globule a. Let the instrument 

 now be placed in a horizontal position, and the entire globule 

 of mercury will instantly quit the open extremity c, of the 

 tube b, leaving the tube exactly filled with that fluid, and the 

 glubule will then take the position c-, Fig. 3, when the instru- 

 ment rests on the edge of its scale. If, from the instant 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 eva- 

 porating 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 

 evaporating fluid is kept applied to their bulbs. The atmo- 



