On a Maximum and Minimum Mercurial Thermometer. 229 



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the other. Bring the mag- 

 nets up to the index in such a 

 manner, that, while the poles 

 of the one bear against the 

 side of the glass tube, the 

 overlapping poles shall lie over 

 the tube so as to be in front 

 of the index : the index will 

 now follow the motion of the 

 magnets, and it may thus be 

 brought down to the surface 

 of the mercury. In order to 

 bring the horizontal index to 

 the extremity of the spirit 

 column, all that is necessary 

 is to incline the horizontal 

 tube a little downwards by 

 pressing on the end. 



The indices being now set 

 and the instrument in adjust- 

 ment, let us suppose the tem- 

 perature to rise ; the mercurial 



column will push the vertical index up, but this index will remain in its 

 place when the mercury again falls, and will therefore denote the 

 maximum temperature reached. On the other hand, let us suppose 

 the temperature to fall. "The mercury in falling is followed by the. 

 spirit column propelled by the air behind it. The spirit column, again, 

 will, on its edge coming in contact with the end of the horizontal index 

 draw the index with it into a position, where it will remain when 

 the mercury again rises. This index will therefore register the 

 extreme minimum point which the spirit column has reached ; but, 

 by the principle of graduation, this will correspond with the mini- 

 mum point reached by the mercurial column. 



Let us now suppose that a small portion of the spirit column has 

 become separated, and lodged itself in the extremity of the tube. 

 The principle of graduation will immediately enable us to discover 

 this, by a want of correspondence being produced in the readings 

 of the mercurial and of the spirit column. If, for instance, before 

 the separation, the mercury read 50°, and the horizontal extremity 

 of the spirit column also 50°, it is clear that, after the abstraction 

 of spirits has taken place, the horizontal column will read lower. 



We have thus a check upon this possible source of error, which 

 we have not in the ordinary minimum thermometer. Indeed, it is to 

 all intents a mercurial minimum thermometer that we are now de- 

 scribing, the spirits serving merely as a vehicle for the indices. It 

 will be remarked, that were both columns capable of acting in a hori- 

 zontal position, there would be no necessity for the bend, and the 

 instrument would be more portable ; but in this position it is found 

 that there is danger of the spirits becoming mixed with the mercury, 

 and thus interfering with the action of the instrument. Should this 

 ever be brought about by travelling, or any other cause, a smart jerk 



