METEOROLOGICAL INSTRUMENTS. 113 



Fig. 591. 



Self - Registering Thermometer for extreme Heat and Cold. 

 (Fig. 591, as above.) In meteorological observations it 

 is of great importance to ascertain the limits of the range of 

 the thermometer in a given period of time, as during a day 

 or night while the observer is absent. The self-registering 

 thermometer most generally used for this purpose consists 

 simply of two thermometers, one mercurial, and the other 

 of alcohol, having their stems horizontal ; the former has for 

 its index a small steel wire, and the latter a minute thread 

 of glass, having its two ends formed into small knobs. 



The wire lies in the vacant space of the mercurial thermo- 

 meter, and is pushed forward by the mercury whenever the 

 temperature rises, and pushes that fluid against it ; but 

 when the temperature falls, and the fluid retires, this index 

 is left behind, and consequently shows the maximum. The 

 other index of glass lies in the tube of the spirit thermometer 

 immersed in the alcohol ; and when the spirit retires, by 

 depression of temperature, the index is carried along with it, 

 in apparent contact with its interior surface ; but on increase 

 of temperature, the spirit goes forward and leaves the in- 

 dex, which, therefore, shows the minimum of temperature 

 since it was set. As these indices merely lie in the tubes, 

 their resistance to motion is altogether inconsiderable. The 

 steel index is brought to the mercury by applying a magnet 

 on the outside of the tube, and the other is duly placed at 

 the end of the column of alcohol, by inclining the whole 

 instrument. 



Price, $4.00. 



" with long cylindrical bulbs and magnet 



to move indexes, - $4.50 and $5.00. 



" mounted singly, mercurial or day ther- 

 mometer, - $2.50. 



" " " alcohol or night ther- 



mometer, .... $2.00. 

 10* 



