SELF-REGISTERING THERMOMETERS. 5 



most cases the moving will not be required. After the first year 

 the zero point changes little, and practically, when exposed only 

 to atmospheric influences, may be considered permanent.] 



SELF-REGISTERING THERMOMETERS. 



Placing. These two thermometers, indicating the maxima 

 and minima, are to be placed beside the common thermometer, 

 in a horizontal position, with the bulbs opposite and free, on two 

 small perpendicular supports uniting the two bars, as shown in 

 Fig. 1. 



Reading. For the reading, place the eye in such a position 

 that the visual ray may be perpendicular to the extremity of the 

 index; enter the indications with the fractions of degrees, if 

 there are any, and, after having verified them again, bring, back, 

 by means of the magnet, the indexes of the two thermometers 

 to the summit of their respective columns. 



Verification. Compare the indications of the two thermome- 

 ters frequently, and especially the spirit thermometer, with those 

 of the common thermometer ; verify the zeros at least twice a 

 year, and, if there is a difference, adjust the zero anew, if the 

 instrument permits, to eliminate the correction, as has been 

 stated above for the simple thermometer, or take this correction 

 into account in the register. 



[The maximum thermometer is subject to derangement by the 

 mercury getting to the side of the steel index and wedging it 

 fast. When such is the case, put the bulb in ice, if it is neces- 

 sary to bring the mercurial column so low, or cool it sufficiently 

 to get all the mercury down that will pass the index ; then move 

 the magnet along the tube with a slight knocking or jarring 

 motion, and try to get the index into the chamber at the top of 

 the stem. If you get the index free of the wedge, but with mer- 

 cury above it, heat the bulb until all the disjointed mercury and 

 index are driven into the chamber, then keep the index up by 

 the magnet, and the mercury will go back as the bulb cools. 

 The great point of attention is to get and keep the index free of 

 the wedge. The mercury being above is of little consequence, 

 as it can readily be heated up into the chamber ; in doing this, 

 most watchfulness is required in not suffering the index to wedge 



