1840.] On the Construction and use of Portable Barometers. 263 



to the wood mounting by a screw. In consequence of the shrinking in 

 the wood between these points, the scale had bowed out, and the person 

 who had used it had therefore removed the screw at the upper end ; and 

 it could be easily seen, from the hole of the screw which was worked into 

 a piece of brass no longer coinciding with the hole in the scale, that the 

 wood within the length of this 14 inches had shrunk in length full one- 

 twentieth of an inch, and therefore the v/hole instrument has decreased 

 in leng<^h one-tenth of an inch— yet the scale very absurdly professes to 

 give the height of the mercury to the 500th part of an inch. 1 am un- 

 able to see any advantage in this plan for a portable barometer, and on 

 the contrary have pointed out undeniable disadvantages. 



4. The object to be gained in a portable barometer is principally to 

 prevent the column of mercury striking too heavily against the top of the 

 glass tube, which may, I think, be easily gained, by making the cistern 

 wholly of iron ; in the bottom an iron screw may be placed, the point of 

 which has a piece of India rubber attached to it, and which being screw- 

 ed up will close the bottom of the tube. A very fine capillary passage 

 should be worked down the point of this screw, and out laterally, so as 

 to allow of the gradual descent of the mercury, in case the instrument 

 should be placed upright, and also of the expansion in the tube. Such 

 an instrument could only be destroyed by inverting it witii a jerk after 

 being held upright. 



5. Guy Lussac's plar> of the syphon barometer is the best portable 

 instrument I am acquainted with, and I consider the instrument here 

 described as constructed for me by Mest,= s^ W. and J, Jones of Holbourn, 

 London, to be the best adapted for use in India. 



6. The case is constructed solely of brass, it is 36| inches long, and 

 is formed of two brass tubes, about seven hundred parts of an inch in 

 thickness, joined together by a short brass cylinder between them, into 

 which they are both screwed, th ' upper being seven -tenths of an inch 

 in diameter, and the lower nine. The top and bottom are closed with 

 screwed plugs with rings. In the front and back of each of these tubes, 

 a slit, 9 inches long, and a quarter of an inch wide, is worked, through 

 which the glass tube is visible. On the edges of these slits the divisions 

 are laid off upwards and downwards from a central point. On both the 

 upper and lower tubes, two short thin brass cylinders, each -| of an inch 

 long, slide freely. On the lower part of the upper of ihese pieces, on 

 each tube, a milled ring wsorks in a groove, and the upper part of the 

 other cylinder, having an external screw cut on it, works into an internal 

 screw cut in the brass ring, by which on turning round the milled ring, 

 the lower portion of each of these cylinders is raised or lowered about 



