264 On the Construction and use of Portable Barometers. [Oct. 



three-tenth of an inch, so as to allow of an adjustment in this manner to the 

 upper surface of the mercurial column, as seen through the slit. In each 

 of the lower pieces of these sliding cylinders, a portion is cut out ^ au 

 inch long and a I wide. The sides of the notch thus formed coincide 

 with the line of the divisions, and on one of these sides the vernier is 

 cut ; the bottom of the lower piece of the sliding cylinder bein^ the bot- 

 tom of the vernier, and being brought as a tangent to touch the top of 

 the mercury. The back and front part of the bottom of this piece, being 

 both made to apparently touch the mercury, prevent any error from pa- 

 rallax by the uncertain position of the eye in taking the observation. 



7. The glass tubes are formed in four parts, the upper is 11 inches 

 long, and at bottom is drawn out in(o a capillary qiiill 2 inches long. A 

 piece of tube, of a little larger diameter, and four inches long, is soldered 

 to this tube, above the contraction, so that the capillary quill projects 

 free two inches into the centre of this short piece of tube. To the other 

 end of this short piece is soldered the rest of the tube, which is bent 

 up as usual in syphon barometers, and the recurved end is finished as in 

 Guy Lussac's plan with a small lateral hole, over which a piece of wash 

 leather is tightly bound, or a capillary glass quill 3 inches long is insert- 

 ed into the end and soldered, and a piece of wash leather is tied over 

 the outer orifice. If the tube is of the latter construction, it is neces- 

 sary to be careful after setting up the instrument to oscillate the mer- 

 cury up to the summit of the tube, so as to draw out any small portion 

 of mercury from the quill, which would make the mercurial column 

 stand too high. By the capillary quill in the centre of the tube no por- 

 tion of air can get into the top, and the tube is in no v/ay affected by 

 any portion of air which may get into the lower part. As thick tube? 

 cannot be sol dered together in the above manner, these tubes are neces- 

 sarily slight, but packing with India rubber, so as to support them 

 closely, for which Messrs. Jones have my directions, they are rendered 

 quite as safe in carriage as any others. The price charged for these 

 instruments by the makers was 7 guineas each, and one very great ad- 

 vantage in them is that fresh tubes can be inserted in a few minutes, 

 which no other kind of portable barometer will admit of. 



8. A very great advantage in syphon barometers is that the capil* 

 lary action of the tubes is neutralized, which, in India, where but few 

 opportunities of comparing barometers occur, is not to be lightly re- 

 garded. Experiments have not yet decided what the true capillary 

 action in barometer tubes really is, and in Daniel's Essays it is plain- 

 ly shown that capillary action in the same tube differs according to 

 circumstances. 



