252 Mr Blackadder on Meteorological Instruments which 



tremity of the fluid in the stem of the instrument ; so that the 

 former shall accompany the latter in all its movements, until 

 the instant arrive when we wish to determine the existing tem- 

 perature- At this instant the index is so acted upon as to re- 

 main 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 adjustment 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 colouring matter usually added to spirit-thermometers, is 

 in this instance of no use, and would be injurious. For, after 

 a time, the colouring matter is partially deposited, and parti- 

 cles of this getting into the stem of the instrument, would in- 

 terrupt the movements of the index. It is for the same rea- 

 son 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. Mr Blackadder had, on one occasion, much trou- 

 ble in adjusting an index, and, at length, discovered, that the 

 whole had arisen from a very minute particle of colourless 

 glass, which had by some accident got into the stem. With 

 proper care and attention, however, nothing is more simple 

 than the construction of a good and perfectly accurate spirit- 

 thermometer, for meteorological purposes. Nothing, at the 

 same time, is more i-arely to be met with; for euch instru- 

 ments, as usually made, are exceedingly inaccurate, and alto- 

 gether unfit for scientific purposes. 



When a thermometer has been constructed in the way now 

 described, all that is necessary to keep the index constantly and 

 exactly at the summit of the fluid, whatever change of tem- 

 perature may take place, is to invert the instrument, and re- 

 tain it either in a perpendicular or somewhat inclined position ; 

 the attraction of the fluid to the index being quite sufficient 



* The bore of the tube admits of being so very minute, that the diffi- 

 culty of readily distinguishing the index is the chief obstacle, and hence 

 the bulb does not require to exceed three or four tenths of an inch in dia- 

 meter. 



