218 TENTH ANNUAL REPORT OF 



SELF-REGISTERINa THERMOMETERS. 



Pladiuj. — These two thermometers, indicating the maxima aflcl 

 minima, are to be placed beside the common thermometer, in a hori- 

 zontal position, with the bulbs opposite and free, on two small per- 

 pendicular 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 thermometers fre- 

 quently, 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 necessary 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 mercury 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 in- 

 dex free of the wedge. Themcrcurybeingabovcjis 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 by 

 the driving mercury. If the index is so wedged that it cannot be 

 moved by these methods, then take the thermometer steadily in the 

 hand, and swing it quickly, as if you wished to throw the mercury 

 into the chamber at the top ; tbe index Avith more or less mercury 

 will be found in the chamber : if not, repeat 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 magnet with the spirit-thermometer, the same care is 

 necessary as with the mercurial, as the index may 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 re- 

 quires attention, also, in this particular. The vapor above the spirit 

 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 mer- 

 curial one, this may be suspected and looked for. When so found, 



