Show thermometer deterioration even thermometers become old and consequently inaccurate with age mercurial instruments reading too high and spirit instruments too low in tile the former case the bulb appears to shrink thus forcing the quicksilver too far up the stem this gradual shrinkage Is s su supposed to be due dua to the tact fact that the external pressure on the bulb may be considerably higher than the internal pressure the air ns as far as aa possible having been removed before the glass la Is sealed scaled on the other hand the spirit thermometer Is sealed with the bulb covered in a freezing mixture in order to lock up in the glass as much air as possible the instrument thus starts with the internal pressure which in time appears to be reduced either by expansion of the glass under the internal pressure or by leakage thic american |