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The indices being DOW set and the instrument in adjustment, let 

 us suppose the temperature 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 tem- 

 perature 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 

 or two of the instrument will join the separated columns and put all 

 right. 



The instrument is thus constructed : The vertical tube, including 

 the bulb, is first made and filled with mercury to the proper height, 

 and the magnetic index is introduced ; then the horizontal tube is 

 joined, and the spirits of wine and the horizontal index are introduced. 

 The bulb is then placed in a freezing mixture, in order that the 



