THE SMITHSONIAN INSTITUTION. 



223 



the observation is to be made in the night, place the lamp before, and 

 not behind the instrument, and somewhat higher tlian the eye ; and 

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

 top of the column, a piece of white paper, which reilects the light. 



Fig. 5. 



e 

 m 



m 



In the barometers with an ivory point, as the Fortin, Newman, 

 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 coincide exactly with its 

 image reflected below by the mercury. This method may be very 

 good when the surface of the mercury is perfectly pure and brilliant ; 

 but this is very rare; it is generally dimmed by a slight layer of' 

 oxide, which makes the coincidence of the point with its image un- 

 certain. It is safer to judge of the contact in a different manner. 

 From the moment when the point does more than touch the surface, 

 it forms around itself, by capillary action, a small depression, which, 

 breaking the direction of the reflected rays, becomes immediately very 

 easy to discover. It is enough, then, to raise the mercury m as 

 slightly to immerse the point, then to lower it gradually until the 

 little depression disappears. If care is taken to make a good light 

 fall on that portion of the mercury which is under the point, and to 

 use the aid of a magnifier, the adjustment of the point thus made, be- 

 comes not only easy, but very certain, and the errors to which we are 

 liable are almost insensible, for they do not exceed two or three hun- 

 dredths of a millimetre, or a thousandth of an inch. 

 • d. The level being thus adjusted to the zero of the scale, we pro- 

 ceed to observe the height of the summit of the column. Take hold 



