which determine the Indications during Absence. 341 



the summit of a mountain, or tower, he would thus have his ope- 

 rations greatly facilitated, and insured of accuracy. 



The principle of this contrivance applies either to the spirit 

 or mercurial thermometer, and consists in keeping a small index 

 suspended at, or in contact with, the extremity of the fluid in 

 the stem of the instrument ; so that the former shah 1 accom- 

 pany the latter in all its movements, until the instant arrive 

 when we wish to determine the existing temperature. At this 

 instant the index is so acted upon as to remain fixed to its place, 

 while the fluid either passes beyond, or retires below it. 



When a spirit thermometer is used, the bore of the tube, and 

 the weight and form of the index, require attention ; but the ad- 

 justment is not difficult. As to the spirit, there is a certain 

 strength which seems to answer best, and it must be colourless, 

 of some age, and carefully and repeatedly filtered. The colour- 

 ing matter usually added to spirit-thermometers, is in this in- 

 stance of no use, and would be injurious. For, after a time, the 

 colouring matter is partially deposited, and particles of this get- 

 ting into the stem of the instrument, would interrupt the move- 

 ments of the index. It is for the same reason that old spirit 

 and frequent filtration are requisite ; for if the spirit is new, and 

 if not frequently and carefully filtered, small whitish flocculi, or 

 minute fibres, may be seen suspended in the fluid, from which 

 interruption to the index is liable to take place. I had, on one 

 occasion, much trouble in adjusting an index, and, at length, dis- 

 covered, that the whole had arisen from a very minute particle of 

 colourless glass, which had by some accident got into the stem of 

 the instrument. With proper care and attention, however, no- 

 thing is more simple than the construction of a good and per- 

 fectly accurate spirit-thermometer, for meteorological purposes 

 Nothing, at the same time, is more rarely to be met with ; for 



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