THE THERMOMETER. 



79 



every reader will be able to read the records from foreign 

 upon the Centigrade and Reamur instruments, which are 

 all based upon the theory that heat expands liquids. 



[We used to hear the expression, " Heat expands, 

 and cold contracts," but we trust that all our readers 

 have now learnt that there is no such thing as cold. 

 It is only a negative term. We feel things cold because 

 they extract some warmth from our ringers, not because 

 the substances have no heat] 



Thermometers are made of very fine bore glass 

 tubes. One end has a bowl, or bulb, the other is at first 

 open. By heating the bowl the air in the tube is driven 

 away by the open end, which is quickly dipped in a 

 bowl of mercury. The mercury will then occupy a 

 certain space in the tube ; and if it be heated till the 

 liquid boils, all the air will be driven out by the mercurial 

 vapour. By once again dipping the tube in the quick- 

 silver the glass will be filled. Then, before it cools, close 

 the open end of the tube, and the thermometer is so far 

 made. Having now caught our thermometer we must 

 proceed to mark it, which is an easy process. By 

 plunging the mercury into pounded melting ice we can 

 get the freezing point, and boiling water will give us 

 the boiling point The intermediate scale can be then 

 indicated. 



If mercury and glass expanded equally there would 

 be no rise in the latter. Extreme delicacy of the ther- 

 mometer can be arrived at by using a very fine tube, par- 

 ticularly if it be also flat 



The freezing point in Fahrenheit's scale is 32; in 

 the Centigrade it is o, and the boiling point 100. This 

 was the scale adopted by Celsius, a Swede, and is much 

 used. Reamur called the freezing point o, and the 

 boiling point 80. There is another scale, almost obsolete, 

 that of Delisle^ who called boiling point zero, and 

 freezing point 150. 



There is no difficulty in converting degrees on one 

 scale into degrees on the other. Fahrenheit made his 

 zero at the greatest cold he could get; viz., snow and 

 salt. The freezing point of water is 32 above his 

 zero. Therefore 212-32 gives 180 the difference 

 between the freezing and boiling points of water. So 

 1 80 Fahr. corresponds to 100 Cent, and to 80 

 Reamur, reckoning from freezing point. 



The following tables will explain the differences : 



countries noted 



Fig- 79- Thermometer. 



