30 



Plan for Metallic-tube Barometers. 



[Jan. 



It is essential that this enlarged portion of the tube be of uniform 

 diameter or bore — this point must be ascertained before the top L be 

 fixed on. The instrument is to be held perpendicularly, the stop cock 

 K which cuts off the communication between the two limbs being 

 closed, and the glass limb properly and tightly fixed on, mercury is 

 poured into both limbs. Into the shorter as much mercury is intro- 

 duced as will bring the top of it within the range of the scale. The 

 longer one is to be filled within a line or two of the top. In this a float 

 carrying a vernier is to be introduced, near the vernier a scale of 

 inches and parts of an inch. This arrangement being made, the stop 

 cock is turned so as to allow a certain quantity of mercury to flow from 

 t\fi longer into the shorter limb. The descent of the float in the for- 

 mer ought to have a certain proportion to the length of that in the 

 shorter limb— the proportion as above stated being inversely as the 

 square of the diameters. 



If this proportion (which by means of the respective scales and ver* 

 niers can easily be known, to of an inch) hold good, the upper part 

 M is perfect as to bore. The lid L is now to be fastened on airtight- 

 it is primarily, however, ground to fit exactly, and has a shoulder all 

 round, corresponding to one on the top of the tube. Moreover it enters 

 this a short way, the exact distance being marked outside by a line 

 or dot. 



The brass tube E D has a slit on opposite sides, and extending its 

 whole length— the magnesian cement beneath being removed, the 

 fluctuations of the mercury in the glass tube are seen. /The usual 

 scale of inches is affixed to this tube, either on a separate piece of 

 brass, which can be moved to admit of adjustment, or engraved on the 

 tube itself. The vernier G F can be moved up and down the tube, or 

 fixed.at any part, by means of the screw G, after which it can be 

 brought to cut the height of the mercury exactly by means of the tan- 

 gent screw L. 



The instrument is now ready to be filled with mercury. The glass 

 limb is first to be removed, then the plug of the stopcock K. Mercury 

 is now poured into the long limb until this is half full, when the whole 

 is placed in a sand bath and the mercury made to boil. "When this is 

 effected, more mercury is poured in until the long limb is filled, wnen 

 the plug K is replaced —turned so as to prevent the escape of mercury, 

 of which the short portion C is next filled and a small funnel, repre- 

 sented by the dotted lines, fixed on the aperture, so that the mouth will 

 be uppermost when the long limb is placed lowermost in the sand bath 

 to which the whole is conveyed, and the mercury boiled, K being pre- 

 viously opened. When cold, the stopcock is turned to cut off commu- 

 nication—the funnel removed, and the glass limb applied. Mercury 



