which determine the Indications during Absence. 343 



After thus fully detailing the principle, it will not be neces- 

 sary to enter very particularly into the subsidiary details. The 

 particular form or construction of the instrument being immate- 

 rial ; it may be modified in many ways, agreeable to the particu- 

 lar views of each individual. 



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

 higher temperature than that of the air, is the aqueous vapour 

 originating from the flame of a lamp ; and, in the coldest stormy 

 weather, the flame does not require to be larger than that pro- 

 duced by, at most, two small cotton threads immersed in oil. 

 When gas is at command, that is doubtless the most convenient 

 combustible, as a minute flame can be kept up almost intermi- 

 nably, and without requiring any attention. But it is not diffi- 

 cult to construct a lamp for burning oil, so as to answer every 

 desirable purpose, and not requiring inspection or adjustment 

 once in twenty-four or more hours. 



I may here notice, that a lamp of very simple construction, 

 can be made to burn oil without any wick, giving out an intense- 

 ly white light, and furnishing so regular a supply of heat, that a 

 thermometer on the outside of a window, with which it may be 

 connected, shall not vary half a degree in the course of many 

 hours. 



When a mercurial thermometer is used, the difference is, 

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

 sition ; 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 instrument 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 reservoir, some evaporating fluid. 

 On some occasions, as in a very humid state of the atmosphere, 



VOL. x. P. n* x x 



