12 



BAROMETER. 



not to confound the slight line of light which marks the oppo- 

 site edge of the cistern, with the light reflected by the surface 

 of the mercury against the inner walls ; the former is always 

 sharp and well defined ; the latter vague and indefinite. When, 

 before adjusting the level, the mercury is higher than the upper 

 edge, it is necessary to begin by lowering it beneath the level 

 (see Fig. 4), so as to leave an interval of light, which is then 

 gradually shut out, as has been described. When the observa- 

 tion is to be made in the night, place the lamp before, and not 

 behind, the instrument, and somewhat higher than the eye ; and 

 if the wall itself is not light enough, place behind the cistern, 

 or the top of the column, a uiece of white paper, which reflects 

 the light. 



Fig. 4, 



Fig. 5. 



In the barometers with an ivory point, as the Fortin, New- 

 man, and Green barometers, the extremity of this point is the 

 zero of the scale, which must be brought into exact contact with 

 the surface of the mercury. We commonly judge that this takes 

 place when we see the actual rounded summit of the point co- 



