222 TENTH ANNUAL REPORT OF 



is closed by a movable cover, and which protects it from exter- 

 nal injuries, from dust, and from the direct radiation of the warm 

 bodies, or the currents of air from the wdndow, and diminishes 

 the effect of the too sudden variations of temperature. When it - 

 is to be observed, the barometer is taken bj^ the upper end of the 

 tube, and the suspending ring is made to slide towards the end of the 

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

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

 rial column, as Avell as the surface of the mercury in the cistern, are 

 completely lighted, and the reading becomes easy and certain. More- 

 over, the slight oscillating movement impressed 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. 



Observation. — 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 impiess 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. 



h. Note the degree and the tenths of degrees of the thermometer 

 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 surface of 

 the mercury to its constant level. In Green's first barometers, the 

 metallic envelope of the cistern is pierced through, (oo',) and alloM^s 

 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, (I I',) 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 mercury 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 vertical- 

 ity, if it had been accidentally deranged, and placing the eye exactly 

 at the height of the mercury, 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 not 

 to confound the slight line of light which marks the opposite 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 



