BAROMETER. 11 



hook. The instrument is then in the full light of the window, in 

 front of which the observer places himself; the summit of the 

 mercurial column, as well as the surface of the mercury in the 

 cistern, are completely lighted, and the reading becomes easy 

 and certain. Moreover, the slight oscillating movement im- 

 pressed on the instrument, by changing its place, breaks the 

 adherence of the mercury to the glass, and thus prepares a good 

 observation. After the reading, the barometer is again slipped 

 gently into the box, and this is closed. 



Ohservation. — The different operations of the barometer of 

 constant level should be made in the following order : — 



a. Before all, incline the instrument gently, so as to render 

 the mercurial column very movable ; then, after having restored 

 it to rest, strike several slight blows upon the casing, in such a 

 manner as to impress on the mercury gentle vibrations. The 

 adherence of the mercury to the glass will thus be destroyed, 

 and the column will take its true equilibrium. 



b. Note the degree and the tenths of degrees of the thermome- 

 ter attached to the instrument ; for it will be seen that the heat 

 of the observer's body soon makes it rise. 



c. Bring, by means of the adjusting screw (Fig. 4), the sur- 

 face of the mercury to its constant level. In Green's first baro- 

 meters, the metallic envelop of the cistern is pierced through 

 (o o'), and allows the surface of the mercury contained in the 

 glass cistern to be seen. The plane which passes through the 

 upper edge (e e') of this opening is the true level, or the zero of 

 the scale, to which the surface of the mercury must be restored. 



For this, take hold, with the left hand, of the lower edge of 

 the cistern {II'), taking great care not to disturb its vertical 

 position ; apply the right hand to the adjusting screw (s), and, 

 turning it gently, bring by degrees the level surface, of the mer- 

 cury to the upper edge (e e') of the opening of the cistern, until 

 there remains between the two only an almost imperceptible line 

 of light, as in the Fig. 5 (e e'). Then leave the instrument to 

 itself, to re-establish its verticality, if it had been accidentally 

 deranged, and placing the eye exactly at the height of the mer- 

 cury, examine whether the contact is exact. For this operation, 

 it is important to have a good light ; the cistern ought to be 

 placed higher than the lower edge of the window, so that the 

 light may reach it directly. It is necessary also to take care 



