1840.] On the tonstruction and use of Portable Barometers. 267 



verted, when any bubbles of air in the top will ascend to the open 

 end, the water in the tube assisting its escape upwards. The tube is then 

 to be completely filled with mercury, and allowed to stand five minutes, 

 for the mercury to settle down. On setting it up now in the mercury, 

 if the operation has been properly done, it will be found that the mercury 

 will not fall in the tube, unless the tube is jarred heavily against the bot- 

 tom of the cup, shewing thereby that the air has been completely ex- 

 tracted. Even when dirty, the tubes answer nearly as well when thus 

 filled. Quilled tubes as before described may be thus used, and they 

 are easier freed from air than the common plain tubes. As they 

 are not required to be heated it is unnecessary to solder the 

 parts together, and the upper quilled part may be made only six 

 inches in length, and the lower portion may be joined to the upper by a 

 sealing wax joint. When such tubes are required for high altitudes 

 where the mercurial column is less than 24 inches in length, the lower 

 portion of the tube may be shortened, by cracking off a piece, as it is 

 better to keep the top part as short as possible, it being then most easily 

 freed of air. 



12. The instrument which I prefer for measuring the altitude of 

 the mercury is very portable and simple, and Messrs. Jones have the 

 minute directions necessary for making it. The following is a general 

 description of it. It is formed of a solid piece of dry mahogany', 39 inches 

 long, 2 inches wide, and I|- inches thick — along the centre to within 

 2 inches of each end, a groove three-quarter inch broad and the same ia 

 depth is hollowed out. Across the bottom of this groove a piece of 

 plate glass 1 ^ inches wide is let in level with the face of the piece of 

 wood. This glass forms the cistern for the bottom of the tube, and 

 allows the surface of the mercury to be distinctly seen. The tube is 

 laid in this groove loosely with cotton, and a flat piece of brass, which 

 turns on a pivot across the top of the groove, keeps it in its place. 

 The wood on the right side of the groove is cut away to allow a brass 

 bar, six- tenth inch wide, and quarter inch thick, and 32 inches long, to be 

 fitted in there. This bar is divided into inches from a standard scale, 

 at a noted temperature^ and serves to measure the length of the column. 

 It is not fixed, but can be moved up and down about ^ inch by an ad- 

 justing screw, to allow an ivory point which it bears on its lower end 

 to be brought into exact contact with the surface of the mercury in the 

 cistern. The vernier slides on the upper part of this bar, and has an 

 adjusting screw also. The tongue attached to the vernier turns on a 

 hinge joint to give , room for putting in the tube. A cover, formed of a 

 piece of mahogany of the same length and breadth, and ^ inch thick, is 



