70 I'l.v .1. A. HAKKKIl AND P. CHAPPUIS ON \ 



which has l>een optically worki'd inside and out, in order to render it perfectly 

 cylindrical; the freedom from longitudinal stri;e. thus obtained gives great sharpness 

 to the images obtained through it. The upper end is closed by a stopper of glass or 

 metal, pierced with a fine hole. Into this stopper is fastened the end of the capillary 

 tul>e fused to the thermometric reservoir. The stopper, which is ground perfectly 

 cylindrical, enters the tube, which it fits closely, for a length of 25 millims., and is 

 fixed in position by a very thin layer of Canada balsam, thus forming a perfect joint. 



The lower part of the stopper is plane and well polished, and carries at its centre a 

 very fine steel point 0'5 millim. long, which serves as an index mark to which the 

 mercury may be accurately adjusted. To avoid all displacement of the stopper and 

 tube in their support, a brass collar is fixed in a groove ground in the stopper, and 

 this is firmly screwed to the iron support by the clamp b. The piece of bron/e 

 carrying the manometer tube also serves to maintain the position of the lower end of 

 the scale, and to carry the vernier, whose zero thus occupies an invariable position 

 with regard to the steel point in the manometer tube. The closed limb of the mano- 

 meter is so arranged on its support that its axis is in the same vertical line as tin- 

 point in the upper chamber of the barometer. The scale remains vertical for all posi- 

 tions of the sliding supports of the barometer and manometer. These conditions 

 being fulfilled, it is evident that if the distance between the point in the closed branch 

 and the zero of the vernier is once for all known, a single reading of the scale, corrected 

 for the " index error," which is defined later, suffices to give the difference of level 

 between the two points. 



The closed branch of the manometer fits below into a glass T-piece, the horizontal 

 limb of which communicates with a system of tubes serving for the exhaustion and 

 filling of the reservoir. The lower end of this glass tube is bent horizontally forwards, 

 and communicates by a tap with one of the four taps on the open branch of the 

 manometer. 



(d) Measurements of Pressures. 



The disposition of the manometric apparatus permits, as has just been seen, the 

 measurement at any moment of the distance between the two points in the barometer 

 and the closed branch. The communication between the columns of mercury filling 

 the manometer and the barometer reservoir being established, the pressure exerted 

 by the gas on the mercury in the closed branch is balanced by increasing or 

 diminishing the height of the mercury in the open branch, which is effected by 

 raising or lowering the auxiliary reservoir placed on the left. The barometer tube is 

 simultaneously displaced, in order to keep the mercury in the neighbourhood of the 

 point in the barometric chamber. The equilibrium sought is attained when the 

 mercury just reaches at the same time the points in the closed branch and in the 

 barometer chamber. The observation of this adjustment of the mercury is made by 

 means of two small telescopes magnifying about 36 times, sliding vertically on a brass 



