6 SELF-REGISTERING THERMOMETERS. 



by the driving mercury. If the index is so wedged that it can- 

 not be moved by these methods, then grasp the thermometer 

 lirmly in the hand, and swing it quickly, as if you wished to 

 throw the mercury into the chamber at the top ; the index, with 

 more or less mercury, will be found in the chamber : if not, re- 

 peat the swinging until it is there. Then heat up the bulb until 

 the mercury joins that in the chamber, keep the index up by the 

 magnet, and let the mercury, by cooling, go back in unbroken 

 line. 



In using the magnet to move the index up into contact with 

 the mercury, care must be taken not to urge it too strongly, or 

 it may enter the mercury. 



In using the spirit-thermometer, the same care is necessary as 

 with the mercurial, since the index may sometimes be forced 

 out of the spirit, entangling the vapor and the alcohol. When 

 this is the case, the thermometer must be taken down and held 

 vertically ; a few taps or jars will bring the spirit together. The 

 spirit-thermometer requires attention, also, in the following par- 

 ticular. The vapor above the column is apt, in time, to condense 

 at the end of the tube, commonly at the very end. When the 

 spirit-thermometer stands lower than the mercurial one, this may 

 be suspected and looked for. When so found, the thermometer 

 should be taken down and shaken until the alcohol runs down ; 

 it should then be kept in an upright position for some time, to 

 drain. If it is found difficult to shake down the condensed vapor, 

 the end of the tube may be carefully and slowly* heated with a 

 small lamp, or a small rod of heated iron held at a short distance, 

 keeping the bulb and lower part as cold as possible ; the alcohol 

 by vaporization will then condense at the surface of the spirit in 

 connection with the bulb. Occasionally, in cold climates, spirit- 

 thermometers are deranged by the air absorbed by the alcohol 

 becoming free in the bulb at a low temperature. When this 

 occurs, bring the thermometer to as low a temperature as may 

 be convenient ; then hold it in such a position that the air-bub- 

 ble comes to the juncture of the bulb and tube, warm the bulb 

 till all the air is in the tube ; then, by shaking the thermometer, 

 or by gentle knocking, the spirit will flow down, and the air 

 speck come to the top. 



This does not occur in spirit-thermometers that are closed 

 with a vacuum, and the spirit at the time well freed from air. 



