register their Indications during the Observer'' s Absence. 353 



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 accompa- 

 nies 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 temperature is diminished, the index is dragged 

 along by the fluid. Upon this latter property, the Psychrome- 

 ter, or instrument for registering the lowest temperature, was 

 constructed. If, then, we take such a thermometer, and sus- 

 pend 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 in- 

 stant. And if we also make provision, that the instant the in- 

 strument 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 

 time 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. 



The means by which the bulb of the instrument is kept at 

 a higher temperature than that of the air, is the aqueous va- 

 pour originating from the flame of a small lamp ; and, in the 

 coldest stormy weather, the flame does not require to be larger 

 than that produced by at most two small cotton threads im- 

 mersed in oil. When gas is at command, it is douhtless the 

 most convenient combustible, as a minute flame can be kept 

 up almost interminably, and without requiring any attention. 



When a mercurial thermometer is used, the difference is, 

 that, in this case, the instrument is not placed in an inverted 

 position ; and, when it is brought into a horizontal position, 

 the bulb, instead of being kept at a higher, must be kept at a 

 lower temperature than that of the air. This can readily be 

 effected, by providing the means for supporting a continual 

 evaporation from the surface of the bulb. When the instru- 

 ment receives its horizontal position, the bulb is made to 

 come into contact with a soft hair-pencil, of a hollow circular 

 form, through which distils guttatim, and slowly, from a re- 

 servoir, some evaporating fluid. On some occasions, as in a 



