342 MR BLACKADDER on Meteorological Instruments 



such instruments as are usually met with, are exceedingly in- 

 accurate, and altogether unfit for scientific purposes. 



When a thermometer has been constructed in the way I have 

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

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

 ture may take place, is to invert the instrument, and retain it 

 either in a perpendicular or somewhat inclined position ; the at- 

 traction of the fluid to the index being quite sufficient for the 

 suspension of the latter, and for overcoming its friction on the 

 sides of the tube. When, however, the instrument is placed in 

 a horizontal position, the index no longer accompanies the fluid 

 in all its motions ; for if the temperature rises, the fluid passes 

 the index as if no such body were present ; and if the tempera- 

 ture is diminished, the index is dragged along by the fluid. 

 Upon this latter property, the Psychrometer, or instrument for 

 registering the lowest temperature, was constructed. If, then, 

 we take such a thermometer as I have described, and suspend it 

 vertically, and in an inverted position, on a moveable axis, it is 

 obvious, that, by connecting with it a time-piece, we can have it 

 placed in a horizontal position at any given instant. And if we 

 also make provision, that the instant the instrument comes to its 

 horizontal position, its bulb is exposed to a higher temperature 

 than that of the air, it is evident that the index will point out 

 the exact temperature of the air at the tune the instrument was 

 changed from its vertical position, and that it will continue to do 

 so as long as the instrument retains its new position, and has its 

 bulb kept at a higher temperature than that of the air. 



This, however, will be better understood from the inspection 

 of a model, than from description. The instrument is rudely 

 constructed ; but in other respects it is perfectly accurate, ha- 

 ving been in daily use for the last 15 months ; and, during the 

 coldest and most stormy, as well as during the hottest weather, 

 it has always given perfect satisfaction. 



